applied sciences

Article A Passively Q-Switched Holmium-Doped Fiber with Graphene Oxide at 2058 nm

Jinho Lee and Ju Han Lee *

School of Electronical and Computer Engineering, University of Seoul, Seoul 02504, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-6490-2344

Abstract: This study reports a Q-switching-based, 2058-nm holmium (Ho) fiber laser incorporating a saturable absorber (SA) based on graphene oxide (GO). The SA was prepared with a side-polished fiber, while GO particles were deposited onto the fiber-polished surface to realize an all-fiber SA. A continuous-wave thulium-doped all-fiber laser, which was configured with a master-oscillator power-amplifier (MOPA) structure, was constructed as a pumping source. By inserting the fabricated SA into an all-fiber ring resonator based on 1-m length of Ho-doped fiber, Q-switched pulses could readily be obtained at a wavelength of 2058 nm. The pulse width was observed to vary from 2.01 to 1.56 µs as the pump power was adjusted from ~759 to 1072 mW, while the repetition rate was tunable from 45.56 to 56.12 kHz. The maximum values of average optical power and pulse energy were measured as ~11.61 mW and 207.05 nJ, respectively, at a ~1072 mW pump power.

Keywords: holmium-doped fiber; fiber laser; graphene oxide; saturable absorber; Q-switching

1. Introduction Q-switched operating at the ~2-µm spectral region are attractive coherent light  sources for numerous applications such as medicine, LiDAR, material processing, and  gas sensing [1–4]. Compared with other types of lasers, fiber lasers have a range of Citation: Lee, J.; Lee, J.H. A Passively benefits like low heat accumulation, environmental stability, alignment-free operation, and Q-Switched Holmium-Doped Fiber compactness [5]. Depending on the requirement of an external electrical signal source, Laser with Graphene Oxide at Q-switched fiber lasers can be categorized into two groups: passive and active. Active 2058 nm. Appl. Sci. 2021, 11, 407. Q-switching technique is commonly used in most of commercial products of pulsed fiber https://doi.org/10.3390/app11010407 lasers with output pulse widths from nanoseconds to microseconds, due to its features of a high energy output and a controllable pulse repetition rate. Comparatively speaking, Received: 2 December 2020 passive Q-switching technique is still of intensive research interest due to its future potential Accepted: 29 December 2020 associated with a simple configuration that does not require an external modulator. Published: 4 January 2021 One of the common approaches to implement passive Q-switching in a rare-earth- doped fiber-based laser configuration is to incorporate a passive device with a function of Publisher’s Note: MDPI stays neu- saturable absorption into a laser resonator. This type of a passive device is called “saturable tral with regard to jurisdictional clai- absorber (SA)”. Until now, III-V-compound semiconducting materials have been used for ms in published maps and institutio- commercial-grade SAs and such commercial-grade SAs were successfully employed for gen- nal affiliations. erating Q-switched pulses from a laser cavity [6]. However, various limitations were found for the SAs based on III-V-compound semiconducting materials and the limitations include the essential requirement of high-cost, clean-room fabrication facilities and the narrow Copyright: © 2021 by the authors. Li- operating bandwidth. For the purpose of overcoming the issues related to the SAs based on censee MDPI, Basel, Switzerland. III-V-compound semiconducting materials, intensive investigations have been conducted This article is an open access article to find material substitutes. Until now, a range of promising candidates have been reported distributed under the terms and con- and those materials include carbon nanotubes (CNTs) [7–11], graphene [12], graphene ditions of the Creative Commons At- oxide (GO) [13,14], topological insulators (TIs) [15–22], transition metal dichalcogenides tribution (CC BY) license (https:// (TMDCs) [23–32], transition metal monochalcogenides (TMMCs) [33], black phosphorus creativecommons.org/licenses/by/ (BPs) [34–36], gold nano particles [37–39], chromium-doped fiber [40], and MXenes [41,42]. 4.0/).

Appl. Sci. 2021, 11, 407. https://doi.org/10.3390/app11010407 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 407 2 of 10

Among the aforementioned nonlinear absorption materials, graphene has been re- garded as highly promising for high-performance devices in the field of photonics owing to its distinct and outstanding characteristics, such as its no energy bandgap structure, high carrier mobility, fast relaxation, large optical third-order nonlinearity, and optical transparency over a wide spectral band region [43,44]. Therefore, graphene has proven to be useful in many photonic applications such as four wave-mixing [45], optical mod- ulators [46], polarizers [47], and SAs [48–52]. In particular, the use of graphene for the realization of SAs has received significant attention due to its inherent advantages of fast recovery time, large operating bandwidth, and high optical damage threshold. Since Bao et al. first reported that graphene could be used as an effective SA material in 2009 [12], the graphene-based SAs have been intensively investigated for their application to pas- sively mode-locked or Q-switched lasers at various wavelengths [48–52]. Apart from graphene, GO and reduced GO, which are low-cost variations from graphene have also been investigated in terms of saturable absorption [13,14,53,54]. In general, fiber lasers operating in the wavelength band from 1.9 to 2.1 µm can be realized using thulium (Tm)-doped fiber, thulium-holmium (Tm-Ho) codoped fiber, or holmium (Ho)-doped fiber as a medium. It should be noticed that a Tm-doped fiber cavity generates a pulsed laser output at around 1.9-µm wavelengths, whereas a Ho-doped fiber cavity does so at around 2 µm. Until now, most of the investigations in short pulse generation in the wavelength band from 1.9 to 2.1 µm have been focused on the use of a Tm-doped fiber cavity due to easy availability of semiconductor pumping sources at ~0.79 and ~1.56 µm [55–58]. Ho-doped silica fiber produces photon emission over a wave- length region of 2.05~2.17 µm [59] and is pumped by laser sources operating at ~1.1 and ~1.9 µm. One should note that semiconductor-based pump sources are not easily available at wavelengths of ~1.1 and ~1.9 µm [60–62]. Until now, there have been quite a few reports on Ho-doped fiber-based pulsed lasers, and they were based on various pulse formation techniques such as gain-switching [63–65], nonlinear polarization rotation (NPR) [66], and saturable absorption [67]. It should be noted that the previous works on Ho-doped fiber-based pulsed lasers mostly focused on mode-locked pulse generation with various nonlinear saturable absorption materials, such as CNTs [67,68], graphene [69,70], and BP [71]; however, only a few investigations have been reported with respect to Q-switched pulse generation from a Ho-doped fiber cavity with an SA [72–75]. More specifically, Chamorovskiy et al. demonstrated a Q-switched Ho-doped fiber laser incorporating a CNT-based SA [72], Sholokhov et al. reported a Q-switched Ho-doped fiber laser with a heavily Ho-doped fiber SA [74], and Wang et al. demonstrated the use of a heavily Ho-doped fiber SA for the realization of high-power Q-switched pulses from a Ho-doped fiber laser [75]. In this work, a Q-switched Ho-doped fiber laser using a low-cost SA based on GO is experimentally demonstrated. The SA, which has an all-fiber structure, was implemented through deposition of GO particles onto the flat polished surface of a side-polished fiber. In order to pump a Ho-doped fiber gain medium in the laser ring resonator, our built continuous-wave (CW) Tm-doped fiber laser at 1928 nm, which was followed by a power amplifier operating was used. By inserting the GO-based SA in the prepared Ho-doped fiber-based ring resonator, the pulsed output was found to be readily obtainable at a wavelength of 2058 nm due to passive Q-switching. The measured temporal width of the output pulses was 1.56 µs at a 56.12 kHz repetition rate. Both output pulse width and repetition were tunable by changing the pump power. It should be noticed that this study’s constructed laser has an all-fiber ring configuration unlike the Q-switching-based Ho-doped fiber laser incorporating a SA based on CNT, demonstrated by Chamorovskiy et al. in which a Fabry-Pérot cavity was employed with a free-space coupled -type SA [72]. The main goal of this work is to demonstrate the ultimate potential of graphene oxide (GO) as a base material for the implementation of our SA since it is a low-cost alternative to expensive high-purity graphene in terms of saturable absorption. Note that GO-based SAs have not been used for passive Q-switching of a fiber laser operating at the Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 10

Appl. Sci. 2021, 11, 407 3 of 10

based SAs have not been used for passive Q‐switching of a fiber laser operating at the wavelengthwavelength band beyond 22 μµm,m, eveneven if if they they have have been been well-investigated well‐investigated at at wavelengths wavelengths of of1, 1, 1.5, 1.5, and and 1.9 1.9µm. μm.

2.2. Characterization Characterization and and Preparation Preparation of of the the GO GO-Based‐Based Saturable Saturable Absorber Absorber TheThe conventional, conventional, low low-cost‐cost GO GO was was obtained obtained from from a a commercial commercial company company for for this this experimentalexperimental demonstration. demonstration. The The measured measured Raman Raman spectrum spectrum of of the the GO GO particles particles is is illus illus-‐ − tratedtrated in in Figure Figure 11a.a. The D bandband andand GG bandband werewere observedobserved atat 1348 1348 and and 1592 1592 cm cm−11, ,each. each. TheThe D D peak peak is is known known to to be be induced induced by by the the lattice lattice disorder disorder of ofgraphene graphene [76], [76 while], while the the G peakG peak is caused is caused by the by thephonon phonon excitation excitation at the at Brillouin the Brillouin zone zone center center [77]. The [77]. X The‐ray X-raypho‐ toelectronphotoelectron spectroscopy (XPS) (XPS) spectrum spectrum was was also also measured measured and and the thespectrum spectrum in the in theC 1s C region1s region of the of theGO GO particles particles is shown is shown in Figure in Figure 1b. 1Afterb. After curve curve fitting, fitting, it was it was found found that that the fittedthe fitted C 1s C peaks 1s peaks could could be decomposed be decomposed by three by three peaks peaks at ~284.8, at ~284.8, ~286.7, ~286.7, and and~287.9 ~287.9 eV, whicheV, which correspond correspond to groups to groups of C‐ ofC/C=C, C-C/C=C, C‐O, and C-O, C=O, and respectively. C=O, respectively. These peaks These corre peaks‐ spondcorrespond to those to thoseof the of previously the previously reported reported values values [78]. [78].

(a) (b) G band Data D band C-C/C=C C-O C=O Fitted Curve Intensity (arb. unit) (arb. Intensity Intensity (arb. unit) (arb. Intensity

1000 1500 2000 2500 278 280 282 284 286 288 290 292 294 296 Wavenumber (cm-1) Binding Energy (eV)

FigureFigure 1. 1. ((aa)) Raman Raman spectrum spectrum and and ( (bb)) XPS XPS spectrum spectrum (C (C 1s) 1s) of of the the graphene graphene oxide oxide (GO) (GO) particles. particles.

FigureFigure 22aa illustrates illustrates the the linear linear optical optical absorption absorption spectrum spectrum for the for GO the particles, GO particles, which whichwas measured was measured with a with spectrophotometer. a spectrophotometer. The linear The linear absorption absorption spectrum spectrum indicates indicates that thatthe the GO GO particles particles have have a broad a broad absorption absorption band band that that include include the the 2.0~2.1 2.0~2.1 μµmm regime.regime. A A schematicschematic diagram ofof thethe GO-deposited GO‐deposited side-polished side‐polished fiber fiber is shownis shown in Figurein Figure2b. One 2b. One side sideof a of standard a standard single-mode single‐mode fiber fiber (SMF) (SMF) was finelywas finely polished polished to obtain to obtain a side-polished a side‐polished fiber, fiber,which which enables enables us to us induce to induce evanescent evanescent field field interaction interaction with with the deposited the deposited GO particles.GO par‐ µ ticles.The physical The physical distance distance between between the polished the polished surface surface and core and edge core was edge ~5 wasm. The ~5 μ polishedm. The polishedlength of length the fiber of the was fiber ~3 mm,was ~3 and mm, the and length the length covered covered with GO with was GO ~8 was mm. ~8 Themm. used The usedGO has GO a has single a single layer layer with awith thickness a thickness of ~500 of nm. ~500 Note nm. thatNote the that used the GO used has GO no has atomic no atomiclayer structure. layer structure. The insertion The insertion loss (IL)loss of(IL) the of side-polishedthe side‐polished fiber fiber was was ~0.7 ~0.7 dB dB and and its polarization-dependent loss (PDL) was ~0.04 dB. After the GO particles were deposited its polarization‐dependent loss (PDL) was ~0.04 dB. After the GO particles were deposited onto the polished surface, the IL was changed into ~2.8 dB, while the PDL became ~3.4 dB. onto the polished surface, the IL was changed into ~2.8 dB, while the PDL became ~3.4 dB. Appl. Sci. 2021, 11, x 407 FOR PEER REVIEW 4 4of of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 10

GO (b) (b) ~5GO μm (a) (a) ~5 μm Graphene oxide Graphene oxide Slide glass Slide glass

Core CladdingCore Cladding Absorbance (arb. unit) Absorbance (arb. unit) Quartz BlockQuartz Block 1000 1200 1400 1600 1800 2000 2200 1000 1200 1400 1600 1800 2000 2200 Wavelength (nm) Wavelength (nm) Figure 2. (a) LinearFigure optical 2. absorption(a)Figure Linear 2.optical spectrum(a) Linear absorption of the optical GO spectrum particles. absorption of (theb) Schematic GO spectrum particles. diagram of ( theb) Schematic GO of GO particles.‐deposited diagram (b ) sideof Schematic GO‐polished‐deposited diagram fiber. side ‐polished of fiber. GO-deposited side-polished fiber. Next, nonlinearNext, transmission nonlinear measurement transmission was measurement performed was as a performedfunction of as the a inputfunction of the input pulseNext, peak nonlinear powerpulse for transmissionpeak the fabricatedpower for measurement GOthe ‐fabricateddeposited was SA,GO performed ‐usingdeposited an as experimental a SA, function using of an thesetup experimental input in setup in Figurepulse peak 3a. The power incidentFigure for 3a. thebeam The fabricated wasincident coupled GO-deposited beam into was the coupled GO SA,‐based using into SA the an in GO experimental the‐based transverse SA in setup electric the transverse in electric (TE)Figure mode.3a. The For incident(TE) this mode.measurement, beam For was this coupled wemeasurement, used into a thepassively GO-basedwe used mode a SA passively‐locked in the transversefiber mode laser‐locked electric with fiber a laser with a repetition(TE) mode. rate For ofrepetition this~36.94 measurement, MHz rate andof ~36.94 a temporal we MHz used andwidth a passively a temporal of ~711 mode-locked fs width at 1902 of nm. ~711 fiberSince fs at laser an1902 ultrafast withnm. Since a an ultrafast laserrepetition source rate operating oflaser ~36.94 source at MHz a 2.0~2.1operating and a‐μ temporalm at wavelength a 2.0~2.1 width‐μ ofregionm ~711wavelength did fs at not 1902 existregion nm. in Since did our not anlaboratory, ultrafastexist in our laboratory, µ thislaser particular source operating measurementthis particular at a 2.0~2.1- wasmeasurement carriedm wavelength out was at 1.9carried μ regionm with out did atour not 1.9 built μ existm modewith in our our‐locked laboratory, built Tm mode‐ ‐locked Tm‐ µ dopedthis particular fiber laser.doped measurement The fibernonlinear laser. was transmissionThe carried nonlinear out atcurve transmission 1.9 ofm the with GO curve our‐based built of SA, the mode-locked isGO illustrated‐based SA, Tm- in is illustrated in Figuredoped 3b fiber together laser.Figure The with 3b nonlinear its together corresponding transmission with its correspondingfitting curve curve of [79]: the fitting GO-based curve SA,[79]: is illustrated in Figure3b together with its corresponding fitting curve [79]: I I TI() 1 T exp( )  Tns , (1) TI() 1 T exp( )  Tns , (1) −IsatI T(I) = 1 − ∆T · exp( ) − Tns,Isat (1) Isat where T(I) and ΔT are the transmission and modulation depth, respectively. I and Isat are where T(I) and ΔT are the transmission and modulation depth, respectively. I and Isat are thewhere inputT(I‐)pulse and ∆ energyT are the and transmission saturation andenergy, modulation respectively. depth, T respectively.ns represents Itheand nonsaturaI are the‐ the input‐pulse energy and saturation energy, respectively. Tns representssat the nonsatura‐ bleinput-pulse loss. The energy modulationble andloss. saturation The depth modulation and energy, saturation respectively. depth andpower saturationTns wererepresents estimated power the nonsaturable were at ~13.1% estimated and loss. ~12.5 at The ~13.1% and ~12.5 W,modulation respectively. depthW, andrespectively. saturation power were estimated at ~13.1% and ~12.5 W, respectively.

55 55 Unsaturable Loss (a) (a) Power Meter Power (b)Meter (b) Unsaturable Loss 3-dB 50 3-dB 50 Coupler Experiment Coupler Fitting Experiment Fitting 45 Power Meter 45 Modulation Depth: ~13.1% Power Meter Modulation Depth: ~13.1% Mode-locked Mode-locked Saturation Power: ~12.5 W PC VOA 40 Saturation Power: ~12.5 W Fiber Laser PC VOA (%) Transmission 40

Fiber Laser (%) Transmission GO-based GO-based SA SA 35 0 25507510012515035 0 255075100125150 Incident Peak Power (W) Incident Peak Power (W) Figure 3. ((aa) )Measurement MeasurementFigure 3. ( asetup) setup Measurement for for nonlinear nonlinear setup transmission transmission for nonlinear curve. curve. transmission (b) (Measuredb) Measured curve. nonlinear (b nonlinear) Measured transmission transmission nonlinear curve transmission curve of the of pre the‐ curve of the pre‐ preparedpared GO GO-based‐based paredsaturable saturable GO ‐absorberbased absorber saturable (SA). (SA). (PC: absorber (PC: polarization polarization (SA). (PC: controller, controller, polarization VOA: VOA: controller,variable variable optical VOA: optical attenuator). variable attenuator). optical attenuator).

3. Q‐Switching3. of Q a‐ Switching2058‐nm Fiber of a 2058Laser‐nm Fiber Laser 3. Q-Switching of a 2058-nm Fiber Laser The whole laserThe schematic whole laser diagram schematic including diagram a Ho ‐includingdoped fiber a Ho ring‐doped resonator fiber andring a resonator and a The whole laser schematic diagram including a Ho-doped fiber ring resonator and Tm‐doped fiberTm‐based‐doped high fiber power‐based pumping high power source pumping is presented source in Figureis presented 4. Our in laser Figure was 4. Our laser was a Tm-doped fiber-based high power pumping source is presented in Figure4. Our laser constructed withconstructed single‐clad with Ho ‐singledoped‐clad fiber Ho rather‐doped than fiber double rather clad than one, double since cladour tarone,‐ since our tar‐ getedwas constructed laser did not with require single-clad a high Ho-doped output power. fiber ratherWe chose than a double 1900‐nm clad Tm one,‐doped since fiber our targeted laser didgeted not laser require did a not high require output a power.high output We chose power. a 1900-nm We chose Tm-doped a 1900‐nm fiber Tm‐doped fiber laser as a pumpinglaser source as a pumping for the Ho source‐doped for fiberthe Ho considering‐doped fiber the considering fact that the the pump fact abthat‐ the pump ab‐ 55 laser as a pumping source for the Ho-doped fiber55 considering the fact that the pump sorption based on the transition of II87 allows for better quantum efficiency than sorption based on the 5transition→ 5 of II87 allows for better quantum efficiency than absorption55 based on the transition of I8 I7 allows for better quantum efficiency than 55 that of II86 [80]. Taking into account that our prepared SA had a transmission type that of II86 [80]. Taking into account that our prepared SA had a transmission type Appl. Sci. 2021, 11, 407 5 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 10

5 5 that of I8 → I6 [80]. Taking into account that our prepared SA had a transmission configuration,type configuration, our laser our cavity laser cavity was constructed was constructed with a withsimple a simplering structure. ring structure. The all‐fiber The continuousall-fiber continuous-wave‐wave (CW) thulium (CW)‐doped thulium-doped master oscillator master oscillator and power and amplifier power amplifier (MOPA) (MOPA)source consists source of consists a CW ofseed a CW oscillator, seed oscillator, a Tm‐Ho a‐ Tm-Ho-codopedcodoped fiber prestage fiber prestage amplifier, amplifier, and a Tmand‐doped a Tm-doped fiber main fiber‐ main-amplifier.amplifier. The output The output beam beam from fromthe seed the seedoscillator oscillator was wasfirst firstam‐ plifiedamplified with with a preamplifier a preamplifier stage stage and and then then further further amplified amplified with with a a main main power power amplifier amplifier stage. The The output output wavelength wavelength of of the the CW CW seed seed oscillator oscillator was was ~1928 ~1928 nm. nm. The The output output of the of seedthe seed oscillator oscillator was was coupled coupled into into a preamplifier a preamplifier stage stage via via a 1550/2000 a 1550/2000-nm‐nm wavelength wavelength di‐ visiondivision multiplexer multiplexer (WDM). (WDM). For For the the prestage prestage amplifier, amplifier, 1‐m 1-m length length of ofTm Tm-Ho-codoped‐Ho‐codoped fi‐ berfiber an an absorption absorption of of 13 13 dB/m dB/m at at1550 1550 nm nm was was used. used. A commercially A commercially available available 1550 1550 nm erbiumnm erbium-doped‐doped fiber fiber amplifier amplifier (EDFA) (EDFA) was was used used as a aspumping a pumping source source for the for prestage the prestage am‐ plifier.amplifier. The The main main stage stage amplifier amplifier was was comprised comprised a a 793 793 nm nm laser laser diode (LD), a (2(2 ++ 1)1) ×× 1 combiner, andand 3-m3‐m length length of of double-clad double‐clad (DC) (DC) Tm-doped Tm‐doped fiber. fiber. The The cladding cladding absorption absorption of ofthe the DC-Tm-doped DC‐Tm‐doped fiber fiber at at 793 793 nm nm was was ~4.5 ~4.5 dB/m. dB/m. The The corecore andand innerinner claddingcladding diameter were 1010 andand 130 130µ μm,m, respectively. respectively.The The 1928-nm 1928‐nm pumping pumping beam, beam, which which had a had maximum a maximum power powerof ~1.1 Wof was ~1.1 launched W was into launched the Ho-doped into the fiber Ho ring‐doped resonator fiber through ring aresonator 1930/2080-nm throughWDM. a A1930/2080 1-m-long‐nm Ho-doped WDM. A fiber 1‐m‐ (Nufernlong Ho SM-HDF-10/130)‐doped fiber (Nufern was SM used‐HDF as a‐10/130) gain medium was used for as the a gaincavity. medium To ensure for unidirectionalthe cavity. To ensure light propagation, unidirectional an light isolator propagation, was used. an A 90:10isolator output was used.coupler A 90:10 was utilized output tocoupler extract was the utilized laser output, to extract and the the laser output output, pulses and were the obtained output pulses from werethe 10% obtained port through from the a 90:1010% port coupler. through a 90:10 coupler.

1550 nm THDF (1 m) Amplifier

Optical Isolator Optical Isolator Seed Oscillator 1550/2000 nm Output WDM

10% 90:10 Coupler DC TDF (3 m) 793 nm LD PC Optical HDF (1m) Isolator WDM

790/2000 nm combiner

Figure 4.4. ExperimentalExperimental setup setup of of the the Q-switched, Q‐switched, holmium-doped holmium‐doped fiber fiber laser. laser. (THDF: (THDF: thulium-holmium thulium‐holmium co-doped co‐doped fiber, fiber, DC TDF:DC TDF: double-clad double‐ thulium-dopedclad thulium‐doped fiber, fiber, HDF: HDF: holmium-doped holmium‐doped fiber, PC: fiber, polarization PC: polarization controller, controller, WDM: wavelength WDM: wavelength division division multiplexer). multiplexer).

In order to induce lasing lasing from from the the all all-fiber‐fiber resonator, resonator, the pump power was enlarged. enlarged. The CW lasing was observed to start when the pump power reached ~660 mW. mW. Subse Subse-‐ quently, the CWCW lasinglasing modemode waswas convertedconverted into the Q-switchingQ‐switching mode as soon as the pump powerpower arrived arrived at at ~759 ~759 mW. mW. The The measured measured optical optical spectrum spectrum of the of Q-switched the Q‐switched pulses pulsesat a pump at a power pump of power ~759 mWof ~759 is shown mW is in shown Figure 5ina. Figure The peak 5a. wavelengthThe peak wavelength was ~2058 nmwas ~2058and interestingly, nm and interestingly, multiple peaksmultiple were peaks observed were observed in the optical in the spectrum, optical spectrum, implying imply that‐ inggain that competition gain competition among many among longitudinal many longitudinal modes occurred, modes occurred, such longitudinal such longitudinal mode in- modestabilities instabilities in fiber laser in fiber devices laser were devices reviewed were in reviewed [81,82]. This in [81,82]. intermode This gain intermode competition gain competitioncan be associated can be with associated the fact that with no the mode-selective fact that no mode filtering‐selective component filtering was component used in the was used in the laser configuration. Figure 5b illustrated the output oscilloscope traces at Appl. Sci. 2021,, 11,, 407x FOR PEER REVIEW 66 of of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 10

variousvariouslaser configuration. pumppump powerpower Figure levels.levels.5 b ItIt illustrated isis clearlyclearly evident theevident output thatthat oscilloscope stablestable QQ‐‐switchingswitching traces at operationoperation various pump werewere maintainedpowermaintained levels. over over It isa a wide clearlywide range range evident of of pump pump that stable powers powers Q-switching above above the the threshold operationthreshold of wereof ~759 ~759 maintained mW. mW. The The tem overtem‐‐ poralporala wide periodperiod range ofof of thethe pump outputoutput powers pulsespulses above waswas observed theobserved threshold toto decreasedecrease of ~759 whenwhen mW. The thethe pump temporalpump beambeam period powerpower of becamebecamethe output larger.larger. pulses ThisThis was sortsort observed ofof pulsepulse temporal totemporal decrease behaviorbehavior when the dependingdepending pump beam onon pumppump power powerpower became isis aa larger. typitypi‐‐ calThiscal phenomenon,phenomenon, sort of pulse which temporalwhich isis generally behaviorgenerally depending reportedreported inin on passivelypassively pump power QQ‐‐switchedswitched is a typical fiberfiber phenomenon, laserslasers usingusing anwhichan SASA [16,24,35].[16,24,35]. is generally reported in passively Q-switched fiber lasers using an SA [16,24,35]. 0 0 (a) (b) (a) (b) 1072 mW 1072 mW -20 -20 978 mW 978 mW -40 -40 915 mW 915 mW Power (dBm) Power (dBm) -60 821 mW -60 821 mW Intensity (arb. unit) (arb. Intensity Intensity (arb. unit) (arb. Intensity 759 mW 759 mW -80 -802020 2040 2060 2080 2100 -60 -40 -20 0 20 40 60 2020 2040 2060 2080 2100 -60 -40 -20 0 20 40 60 Wavelength (nm) Wavelength (nm) TimeTime ((s)s)

Figure 5. (a) Measured optical spectrum of the output pulses at pump power of 759 mW. (b) Measured oscilloscope traces FigureFigure 5. 5. ((aa)) Measured Measured optical optical spectrum spectrum of of the the output output pulses pulses at at pump pump power power of of 759 759 mW. mW. ( (bb)) Measured Measured oscilloscope oscilloscope traces traces of the output pulses at a variety of pump powers. ofof the the output output pulses pulses at at a a variety variety of of pump pump powers. powers.

Subsequently,Subsequently, the the the average average average optical optical optical power, power, power, pulse pulse pulse energy, energy, energy, and and and repetition repetition repetition rate rate rateof of the the of out out the‐‐ putputoutput pulses pulses pulses were were were monitored monitored monitored while while while changing changing changing the the thepump pump pump power. power. power. The The The results results results are are are illustrated illustrated illustrated in in FigureFigurein Figure 6. 6. The The6. Thepulse pulse pulse width width width decreased decreased decreased from from 2.01 from2.01 to to 2.01 1.56 1.56 to μ μss 1.56 as as the theµ spump pump as the power power pump was was power adjusted adjusted was fromfromadjusted ~759~759 from toto 10721072 ~759 mW,mW, to 1072 whilewhile mW, thethe while repetitionrepetition the repetition raterate changedchanged rate changed fromfrom from 45.5645.56 45.56 toto 56.1256.12 to 56.12 kHz,kHz, kHz, asas shownshownas shown in in Figure inFigure Figure 6a.6a.6 a. TheThe The pumppump pump powerpower power-dependent‐‐dependentdependent pulsepulse pulse widthwidth width changechange could could be bebe attributed attributed toto the the pump pump-inducedpump‐‐inducedinduced gaingain compressioncompression effecteffect [16,35,83]. [[16,35,83].16,35,83]. TheThe pulsepulse energyenergy as as well well as as the thethe averageaverage outputoutput powerpower waswas observedobserved toto increase increase as as the the pump pump power power was was enlarged, enlarged,enlarged, as as shownshown in in FigureFigure6 b.6b.6b. The TheThe maximum maximummaximum values valuesvalues of pulse ofof pulsepulse energy energyenergy and and averageand averageaverage output outputoutput power powerpower were were~11.62were ~11.62~11.62 mW and mWmW 207.05 andand 207.05207.05 nJ, respectively, nJ,nJ, respectively,respectively, each each ateach the atat maximum thethe maximummaximum pump pumppump power powerpower of ~1072 ofof ~1072~1072 mW. mW.mW.Finally, Finally,Finally, a performance aa performanceperformance comparison comparisoncomparison between betweenbetween previously previouslypreviously demonstrated demonstrateddemonstrated Q-switched QQ‐‐switchedswitched Ho- HoHodoped‐‐dopeddoped fiber fiber fiber lasers lasers lasers with with with SAs SAs SAs and and and our our our laser laser laser is presented is is presented presented in the in in the Tablethe Table Table1. It 1. is1. It obviousIt is is obvious obvious from from from the thethetable table table that that that our our our laser laser laser exhibited exhibited exhibited better better better performance performance performance than than than the the the laser laser laser using using using a CNT-based a a CNT CNT‐‐basedbased SA SA SA in ininterms termsterms of of pulseof pulsepulse energy. energy.energy. From FromFrom a perspective aa perspectiveperspective of pulse ofof pulsepulse energy energyenergy scalability, scalability,scalability, the Ho-doped thethe HoHo‐‐dopeddoped fiber fiberfiberSA is SASA the isis best thethe among bestbest amongamong the three thethe three duethree to duedue its high toto itsits damage highhigh damagedamage threshold. threshold.threshold. Apparently, Apparently,Apparently, nanomaterial- nanonano‐‐ materialmaterialbased SAs‐‐basedbased have SAsSAs damage havehave threshold damagedamage issues, thresholdthreshold which issues,issues, are critical whichwhich to areare obtaining criticalcritical hightoto obtainingobtaining energy pulses highhigh energyenergydirectly pulsespulses from a directlydirectly laser cavity. fromfrom aa laserlaser cavity.cavity.

230 16 (mW) Power Output Average 230 16 (mW) Power Output Average 56 2.0 56 (a)220 (b) 15 (a) Repetition Rate 2.0 ( Width Temporal 220 (b) Pulse Energy 15 Repetition Pulse Width Rate ( Width Temporal 210 Pulse Average Energy Output Power 14 54 Pulse Width 210 Average Output Power 14 54 1.9 200 13 1.9 200 13 52 190 12 52 1.8 190 12 1.8 180 11 50 180 11 50 170 10 1.7 170 10 48 1.7 160 9 48  160 9  s) Pulse Energy (nJ) 150 8 1.6 s) Pulse Energy (nJ) Repetition Rate (kHz) 46 1.6 150 8

Repetition Rate (kHz) 46 140 7 140 7 44 1.5 130 6 44 750 800 850 900 950 1000 1050 11001.5 130 750 800 850 900 950 1000 1050 11006 750 800 850 900 950 1000 1050 1100 750 800 850 900 950 1000 1050 1100 Pump Power (mW) Pump Power (mW) Pump Power (mW) Pump Power (mW) Figure 6. (a) Measured repetition rate and temporal width of the output pulses as a function of the pump power. (b) FigureFigure 6. (a) Measured repetitionrepetition rate rate and and temporal temporal width width of of the the output output pulses pulses as aas function a function of the of pumpthe pump power. power. (b) Mea- (b) Measured pulse energy and average output power of the output pulses as a function of the pump power. Measuredsured pulse pulse energy energy and and average average output output power power of the of outputthe output pulses pulses as a as function a function of the of pump the pump power. power. Appl. Sci. 2021, 11, 407 7 of 10

Table 1. Performance comparison between previously demonstrated Q-switched Ho-doped fiber lasers with saturable absorber (SAs) and our laser.

Center Max. Min. Pulse Max. Pulse SAs Wavelength Repetition Reference Width (µs) Energy (nJ) (nm) Rate (kHz) CNT 2097 170 0.32 118 [72] Ho-fiber 2050 190 0.45 7000 [74] GO 2058 56.12 1.56 207.05 This work

4. Conclusions A passively Q-switched 2058 nm laser with an all-fiber configuration incorporating a GO-based SA has been experimentally demonstrated in this study. The all-fiber SA was fabricated with GO particles deposited onto the polished side surface of a standard optical fiber. It was discovered that stable microsecond pulses could be generated at 2058 nm when the prepared SA was inserted into a ring resonator incorporating a Ho-doped fiber gain medium. The minimum pulse width was ~1.56 µs at a ~1072 mW pump power. These experimental results are believed to be a useful part of a database of pulsed Ho-doped fiber lasers, which have not been extensively investigated until now.

Author Contributions: Conceptualization, J.L. and J.H.L.; methodology, J.L.; validation, J.L. and J.H.L.; formal analysis, J.L. and J.H.L.; investigation, J.L.; resources, J.L.; data curation, J.L.; writing— original draft preparation, J.L.; writing—review and editing, J.H.L.; visualization, J.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the 2020 Research Fund of the University of Seoul. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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