nanomaterials

Article Improved Damage Threshold of In2Se3 Saturable Absorber by PVD for High-Power Mode-Locked Er-Doped Fiber Laser

1, 1,2,3, 1,4 1 1,3 Xile Han †, Huanian Zhang †, Shouzhen Jiang , Chao Zhang , Dengwang Li , Quanxin Guo 1, Jinjuan Gao 1 and Baoyuan Man 1,*

1 Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China 2 Institute of Data Science and Technology, Shandong Normal University, Jinan 250014, China 3 Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China 4 Collaborative Innovation Center of Light Manipulations and Applications in Universities of Shandong, Shandong Normal University, Jinan 250014, China * Correspondence: [email protected] These authors contributed equally to this work. †  Received: 20 July 2019; Accepted: 26 August 2019; Published: 28 August 2019 

Abstract: In this study, a double-end pumped high-power passively mode-locked erbium-doped fiber laser (EDFL) was realized by employing a few-layered In2Se3 flakes as a saturable absorber (SA). Herein, the uniform large-scale In2Se3 flakes were synthesized by the physical vapor deposition (PVD) 2 method. The PVD-In2Se3 SA exhibited a remarkable damage threshold of higher than 24 mJ/cm . Meanwhile, the PVD-In2Se3 SA had a modulation depth and saturable intensity of 18.75% and 2 6.8 MW/cm , respectively. Based on the In2Se3 SA, the stable bright pulses emitting at 1559.4 nm with an average output power/pulse energy/pulse duration of 122.4 mW/5.8 nJ/14.4 ns were obtained successfully. To our knowledge, 122.4 mW was the new major breakthrough of mode-locked Er-doped fiber . In addition, this is the first demonstration of the dark-bright pulse pair generation based on In2Se3 SA. The maximum average output power of the dark-bright pulse reached 121.2 mW, which also showed significant enhancement in comparison with previous works. Our excellent experiment results fully prove the superiority of our experimental design scheme and indicate that the PVD-In2Se3 could operate as a promising highly-nonlinear photonic material for a high-power fiber laser.

Keywords: nonlinear optical materials; indium selenide; physical vapor deposition; mode-locked fiber lasers; saturable absorber

1. Introduction Ultrafast fiber lasers have wide applications in a wide variety of areas, such as information transmission, frequency metrology, scientific research and so on. Over the past few decades, ultrashort pulses fiber lasers have received much attention due to their potential applications in industrial manufacturing, environmental monitoring, toxic gas detection, biomedical, defense, optical sensing and optical imaging. Currently, several passively mode-locked techniques have been employed for generating pulsed laser operations. In 2016, Ivanenko et al. reported a mode-locked long fiber master oscillator based on a nonlinear amplifying loop mirror (NALM) with intra-cavity power management, and achieved a record-high pulse energy exceeding 12 µJ[1]. In addition, various types of saturable absorbers (SAs), including semiconductor saturable absorber mirrors (SESAMs) [2,3], single-wall carbon

Nanomaterials 2019, 9, 1216; doi:10.3390/nano9091216 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1216 2 of 12 nanotubes (SWCNTs) [4–6], and graphene [7–9], have been considered the most promising approach to obtain mode-locked pulses. As known, SESAM has been widely employed in a majority of commercial mode-locked fiber lasers to achieve ultrashort pulses. However, SESAM shows some drawbacks, such as a narrow absorption bandwidth, low damage threshold, complex fabrication and packaging and high cost, which have restricted its further advanced development. In addition, SWCNTs have also been widely used as SAs due to their ultrafast excited state carrier dynamic, high damage threshold and low saturation threshold. However, the absorption bandwidth of SWCNTs depends highly on its diameter, which restricts its practical applications. Moreover, graphene is applicable as a broadband SA due to its Dirac-like electronic band structure. In 2009, Zhang et al. reported on a graphene-based erbium-doped mode-locked fiber laser [9]. Since then, a variety of atomically thin layered materials have been extensively investigated and employed as the SA to obtain mode-locked pulses. Transition metal dichalcogenides (TMDs) [10–14] emerged as a novel kind of layered nanomaterial exhibiting the unique layer-number dependent bandgap properties, which have shown a great potential in nanoelectronics, optical sensing and optoelectronics [15]. However, the limited carrier mobility in the monolayer restricts its advanced application. Recently, topological insulators (TIs) (Bi2Se3, Bi2Te3, Sb2Te3)[16–20] with larger modulation depths and high optical nonlinearity are attracting great interest in laser photonics. In 2012, Bernad et al. first demonstrated the saturable absorption of Bi2Te3 [20]. Black phosphorus (BP) [21,22] is beneficial to its application in the field of near infrared optoelectronic materials due to its adjustable direct band gap. Regretfully, broadband SAs were still subject to a low laser damage threshold in practical applications. Recently, indium selenide (In2Se3), a layered semiconducting chalcogenide, has attracted extensive attention for its various exciting physics properties. In2Se3 is considered to be a promising material for the applications in photo-voltaic devices [23], phase change memory [24], and especially in opto-electronics [25]. Previous research indicated that In2Se3 was also available in at least five different phases and crystal structures (α, β, γ, δ, and κ) existing at various temperatures [26]. It is composed of vertically stacked Se-In-Se-In-Se quintuple layers, held together through weak van der Waals forces [27]. Moreover, In2Se3 shows a tunable thickness-dependent optical bandgap ranging from 1.45 to 2.8 eV [28]. Meanwhile, In Se as a direct band gap III VI semiconductor, possesses ultrahigh photodetection 2 3 − responsivity with a fast response time [29]. Therefore, In2Se3 is highly suited to ptoelectronic devices and photonics applications. In the past several decades, the related properties of α-In2Se3 have been extensively studied. However, its nonlinear optical properties have rarely been investigated and are rarely used as SAs in a passively mode-locked operation. At present, a few-layered In2Se3 flakes have been successfully prepared by several methods, mainly including mechanical exfoliation (ME) [29], chemical vapor deposition (CVD) [27], and physical vapor deposition (PVD) [30]. However, a few-layered In2Se3 flakes obtained with the ME method led to an uncontrollable size and random thickness, which limits the nonlinear optical response of In2Se3. Compared with the ME method, the thickness of In2Se3 flakes that prepared by CVD and PVD methods could be controlled accurately. Moreover, the PVD method could reduce defects of the In2Se3 flakes and achieve highly uniform films with large areas, which could improve nonlinear optical properties. Additionally, the In2Se3 flakes prepared by the PVD method possess a higher crystal quality, which could exhibit excellent laser damage threshold. Recently, for optoelectronic devices, Zhou et al. demonstrated that the atomically layered 2 In2Se3 exhibit p-type semiconducting behaviors with the mobility up to 2.5 cm /Vs [31]. Meanwhile, for the ultrafast photonic applications, by inserting α-In2Se3 SA prepared with magnetron-sputtering deposition method into erbium-doped fiber laser (EDFL) systems, Yan et al. obtained soliton pulse with maximal average power/single pulse energy/pulse duration for EDFL of 83.2 mW/2.03 nJ/276 fs, respectively [32]. However, there were no reports on a few-layered In2Se3 flakes with a large-area prepared by PVD method as SA for high-power mode-locked operations. It is well known that the damage threshold of SA materials is an important parameter to decide whether the material can achieve a high-power laser output and be used in the practice of the material. In general, polymer Nanomaterials 2019, 9, 1216 3 of 13 Nanomaterials 2019, 9, 1216 3 of 12 based SAs have been widely integrated into fiber laser systems. However, high peak power in the cavity may lead to changes in the property of polymer and even damage the SAs. based SAs have been widely integrated into fiber laser systems. However, high peak power in the In this report, a double-end pumped mode-locked EDFL based on In2Se3 SA for high-power laser cavity may lead to changes in the property of polymer and even damage the SAs. output is presented. The uniform large-area atomically thin In2Se3 flakes were synthesized on In this report, a double-end pumped mode-locked EDFL based on In Se SA for high-power fluorophlogopite mica (FM) by the PVD method. In addition, by a transfer2 process,3 a few-layered laser output is presented. The uniform large-area atomically thin In2Se3 flakes were synthesized on In2Se3 flakes were directly transferred on the facet of the fiber. This work studied the laser damage fluorophlogopite mica (FM) by the PVD method. In addition, by a transfer process, a few-layered threshold of the PVD-In2Se3 SA, which possessed excellent damage threshold. As is known, the laser In Se flakes were directly transferred on the facet of the fiber. This work studied the laser damage damage2 3 threshold of SESAM (BATOP, SA-1550-35-2ps-x) is 1.5 × 103 μJ/cm2. Compared with the threshold of the PVD-In2Se3 SA, which possessed excellent damage threshold. As is known, the laser SESAM, the laser damage threshold of the In2Se3-FM SA reached as high as 24 mJ/cm2. Meanwhile, damage threshold of SESAM (BATOP, SA-1550-35-2ps-x) is 1.5 103 µJ/cm2. Compared with the the nonlinear optical properties of In2Se3 SA were investigated. It exhi× bited excellent nonlinear optical SESAM, the laser damage threshold of the In Se -FM SA reached as high as 24 mJ/cm2. Meanwhile, performances, such as a large modulation depth2 (13 8.75%) and lower saturable intensity (6.8 MW/cm2). the nonlinear optical properties of In2Se3 SA were investigated. It exhibited excellent nonlinear optical By employing the In2Se3 SA in a bidirectional pumping high-power mode-locked EDFL system, a performances, such as a large modulation depth (18.75%) and lower saturable intensity (6.8 MW/cm2). variety of stable mode-locked pulses were obtained. The average output power was 122.4 mW, By employing the In2Se3 SA in a bidirectional pumping high-power mode-locked EDFL system, corresponding to a single pulse energy of 5.8 nJ. The experiment results fully prove that In2Se3 could a variety of stable mode-locked pulses were obtained. The average output power was 122.4 mW, be a potentially excellent SA for a high-power mode-locked fiber laser application in practice. corresponding to a single pulse energy of 5.8 nJ. The experiment results fully prove that In2Se3 could be a potentially excellent SA for a high-power mode-locked fiber laser application in practice. 2. Preparation and Characterization of the In2Se3 SA

2. Preparation and Characterization of the In2Se3 SA 2.1. Synthesis and Characterization of In2Se3 Flakes

2.1. SynthesisIn our experiment, and Characterization the commonly-reported of In2Se3 Flakes PVD method was employed for preparing high- qualityIn ourIn2Se experiment,3 flakes [30]. the The commonly-reported growing progress PVDis illustrated method wasin Figure employed 1. The for In preparing2Se3 flakes high-qualitywere grown on monolayer FM substrates via van der Waals epitaxy in a horizontal tube furnace (OTL1200). The In2Se3 flakes [30]. The growing progress is illustrated in Figure1. The In 2Se3 flakes were grown onIn2Se monolayer3 power (99.99%, FM substrates Alfa Aesar, via Beijing, van der China) Waals as epitaxy an evaporation in a horizontal source was tube placed furnace at the (OTL1200). constant- temperature zone of the tube furnace heated to 750 °C for 60 min. The vapor was transported The In2Se3 power (99.99%, Alfa Aesar, Beijing, China) as an evaporation source was placed at the downstream by 50 sccm Ar gas with pressure controlled at 15 Pa. The In2Se3 flakes were grown on constant-temperature zone of the tube furnace heated to 750 ◦C for 60 min. The vapor was transported FM substrate placed 12 cm away from the heating center. Finally, the furnace was naturally cooled downstream by 50 sccm Ar gas with pressure controlled at 15 Pa. The In2Se3 flakes were grown on FM substratedown to the placed ambient 12 cm temperature away from thein the heating gas flow center. of Ar. Finally, the furnace was naturally cooled down to the ambient temperature in the gas flow of Ar.

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof thethe physicalphysical vaporvapor depositiondeposition forfor InIn22SeSe33 flakesflakes growth.

Figure2a shows a typical scanning electrical microscope (SEM) (Sigma 500, ZEISS, Oberkochen, Germany) image of the prepared In2Se3 flakes on FM. It is obvious that most flakes exhibit asymmetric hexagonal and truncated trigonal morphology shapes, which are along the horizontal direction. Meanwhile, an energy dispersive spectrometer (EDS, XFlash 6130, Bruker, Germany) was employed for investigating the characteristic of the elemental composition. As shown in Figure2b, the stoichiometric ratio of Se (59.76%) and In (40.24%) is estimated to be 3:2. The structural characterization of the prepared In2Se3 flakes was also tested by a Raman (Horiba HR Evolution) with excitation light of 532 nm. The test result is displayed in Figure2c, where the three peaks at ~108, ~173 and 1 ~205 cm− are attributed to A1 (LO + TO), A1 (TO), and A1 (LO) phonon modes of In2Se3. These Raman features unequivocally indicate the successful preparation of In2Se3 flakes [33]. A blue shift in lattice phonon mode of A1 (TO) might be due to the oxidized caused by the Raman excitation laser [30]. In addition, the crystal structure of the In2Se3 flakes was investigated through X-ray diffraction (XRD) (Bruker D8 ADVANCE, Billerica, MA, USA). Figure2d shows the relatively higher intensity of the (006) peak which indicates that the In2Se3 flakes exhibit a well-layered structure and high crystallinity. As shown in Figure2e, atomic force microscopy (AFM, Bruker Multimode 8, Germany) was used to

Nanomaterials 2019, 9, 1216 4 of 12

determine the thickness of the In2Se3 flakes on FM. Figure2f shows the lateral height profile of the Nanomaterials 2019, 9, 1216 4 of 13 In2Se3 flakes. The thickness of the marked samples ranged from approximately 2.0 to 2.6 nm.

Figure 2. (a) SEM image of the In2Se3 flakes; (b) EDS spectroscopy of the In2Se3 flakes; (c) Raman Figure 2. (a) SEM image of the In2Se3 flakes; (b) EDS spectroscopy of the In2Se3 flakes; (c) Raman spectrum of the In2Se3 flakes; (d) X-ray diffraction of the In2Se3 flakes; (e) AFM image of In2Se3 flakes; spectrum of the In2Se3 flakes; (d) X-ray diffraction of the In2Se3 flakes; (e) AFM image of In2Se3 flakes; (f) Height profile of In2Se3 flakes measured along the white line in (e). (f) Height profile of In2Se3 flakes measured along the white line in (e).

2.2. Preparation and Characterization of In2Se3 SA Figure 2(a) shows a typical scanning electrical microscope (SEM) (Sigma 500, ZEISS, In the experiment, In Se flakes based on a monolayer FM were peeled off into a few layers Oberkochen, Germany) image2 3of the prepared In2Se3 flakes on FM. It is obvious that most flakes exhibitby pyrolysis asymmetric tape. Then,hexagonal the few and layers truncated In2Se 3tr-FMigonal was morphology directly attached shapes, onto which a fiber are end-facet along the for horizontalpreparing direction. an all fiber Me SAanwhile, with a high an laserenergy damage dispersive threshold. spectrometer (EDS, XFlash 6130, Bruker, Germany)The linearwas employed transmission for investigating of In2Se3-FM the from characte 400 toristic 2000 nmof the was elemental measured composition. by using a UVAs /shownvis/NIR inspectrophotometer Figure 2(b), the stoichiometric (Hitachi U-4100). ratio As of shown Se (59.76%) in Figure and3a, InIn2 Se(40.24%)3-FM SA is exhibits estimated a high to transmissionbe 3:2. The of 92.8% at 1560 nm. Here, the linear transmission shows a fringe, which was caused by a spectral structural characterization of the prepared In2Se3 flakes was also tested by a (Horibainterference HR Evolution) caused by with a thin excitation film of light a certain of 532 thickness. nm. The test In addition,result is displayed the nonlinear in Figure absorption 2(c), properties of the In Se -FM SA were investigated by a power-dependent transmission technique. where the three peaks2 at3 ~108, ~173 and ~205 cm−1 are attributed to A1 (LO + TO), A1 (TO), and A1 The representative result is shown in Figure3b. The data for the transmission is fitted by the following (LO) phonon modes of In2Se3. These Raman features unequivocally indicate the successful equation [34]: preparation of In2Se3 flakes [33]. A blue shift in lattice phonon mode of A1 (TO) might be due to the T(I) = 1 ∆T exp( I/Isat) Tns (1) oxidized caused by the Raman excitation laser− [3× 0]. In− addition,− the crystal structure of the In2Se3 flakeswhere wasT is investigated the transmission, through∆ X-rayT is the diffraction modulation (XRD) depth, (BrukerI is D8 the ADVANCE, input intensity Billerica, of the MA, laser, USA).Isat is 2 3 Figurethe saturation 2(d) shows intensity, the relativelyTns is the higher non-saturable intensity loss. of the According (006) peak to the which experimental indicates resultsthat the of In fitting,Se flakesthe saturation exhibit a well-layered intensity, modulation structure and depth high and crystallinity. non-saturable As lossshown is approximatelyin Figure 2(e), atomic 6.8 MW force/cm2 , microscopy18.75% and (AFM, 18.89%, Bruker respectively. Multimode 8, Germany) was used to determine the thickness of the In2Se3 flakes on FM. Figure 2(f) shows the lateral height profile of the In2Se3 flakes. The thickness of the marked samples ranged from approximately 2.0 to 2.6 nm.

2.2. Preparation and Characterization of In2Se3 SA

In the experiment, In2Se3 flakes based on a monolayer FM were peeled off into a few layers by pyrolysis tape. Then, the few layers In2Se3-FM was directly attached onto a fiber end-facet for preparing an all fiber SA with a high laser damage threshold. The linear transmission of In2Se3-FM from 400 to 2000 nm was measured by using a UV/vis/NIR spectrophotometer (Hitachi U-4100). As shown in Figure 3(a), In2Se3-FM SA exhibits a high transmission of 92.8% at 1560 nm. Here, the linear transmission shows a fringe, which was caused by a spectral interference caused by a thin film of a certain thickness. In addition, the nonlinear absorption properties of the In2Se3-FM SA were investigated by a power-dependent transmission technique. The representative result is shown in Figure 3(b). The data for the transmission is fitted by the following equation [34]:

Nanomaterials 2019, 9, 1216 5 of 13

Nanomaterials 2019, 9, 1216 5 of 13 TI()=−Δ×1exp T() − II − T sat ns (1) TI()=−Δ×1exp T() − II − T Where T is the transmission, ΔT is the modulation depth,sat ns I is the input intensity of the laser,(1) Isat is the saturation intensity, Tns is the non-saturable loss. According to the experimental results of Where T is the transmission, ΔT is the modulation depth, I is the input intensity of the laser, Isat fitting, the saturation intensity, modulation depth and non-saturable loss is approximately 6.8 is the saturation intensity, Tns is the non-saturable loss. According to the experimental results of MW/cm2, 18.75% and 18.89%, respectively. Nanomaterialsfitting, the 2019saturation, 9, 1216 intensity, modulation depth and non-saturable loss is approximately5 of6.8 12 2 MW/cm , 18.75% and100 18.89%, respectively. (a) (b) 90 80 100 80 (a) (b) 90 92.8%@1560nm 8075 70 2 80 Isat=6.8MW/cm 60 92.8%@1560nm 75 ΔT=18.75% 70 70 2 50 ITsatns=6.8MW/cm=18.89% 60 ΔT=18.75% Transmission (%) Transmission 40 70 Transmittance (%) 65 50 Tns=18.89% Experiment data In Se /FM 30 2 3 Fitting curve Transmission (%) Transmission 40 Transmittance (%) 65 20 60 Experiment data In Se /FM 30400 600 800 1000 1200 1400 1600 18002 3 2000 0 102030405060 Fitting curve 2 20 Wavelength (nm) 60 Optical Intensity (MW/cm ) 400 600 800 1000 1200 1400 1600 1800 2000 0 102030405060 Wavelength (nm) Optical Intensity (MW/cm2) Figure 3. (a) Linear transmission of the In2Se3-FM saturable absorber (SA) versus the wavelength. (b) The nonlinear absorption property of the In2Se3-FM SA. Figure 3. (a)(a) Linear Linear transmission transmission of of the the In In2Se2Se3-FM3-FM saturable saturable absorber absorber (SA) (SA) versus versus the thewavelength. wavelength. (b) (b) The nonlinear absorption property of the2 In3 Se -FM SA. 3. ExperimentalThe nonlinear Setup absorption property of the In Se2-FM3 SA. 3. Experimental Setup 3. ExperimentalThe experimental Setup setup of the double-end pumped fiber laser was shown in Figure 4. As is shown, a piece of 62 cm high-concentration erbium-doped fiber (EDF, LIEKKI Er-80-8/125) with a The experimental setup of thethe double-enddouble-end pumpedpumped fiberfiber laserlaser waswas shownshown inin FigureFigure4 .4. AsAs isis group velocity dispersion (GVD) of −19.5 ps2/km was used as the gain medium. Two 976 nm laser shown, aa piecepiece of of 62 62 cm cm high-concentration high-concentration erbium-doped erbium-doped fiber fiber (EDF, (EDF, LIEKKI LIEKKI Er-80-8 Er-80-8/125)/125) with a with group a velocitydiodes (LDs) dispersion were (GVD) employed of 19.5 as psthe2/km pump was usedsource. as theTwo gain 980/1550 medium. nm Two wavelength 976 nm laser division diodes group velocity dispersion (GVD) of −19.5 ps2/km was used as the gain medium. Two 976 nm laser multiplexers (WDMs) were used− to couple the pump power into the ring laser cavity. A polarization- (LDs)diodes were (LDs) employed were asemployed the pump as source. the Twopump 980 so/1550urce. nm Two wavelength 980/1550 division nm wavelength multiplexers (WDMs)division insensitive isolator (PI-ISO) was employed to ensure the unidirectional laser operation. Two weremultiplexers used to (WDMs) couple the were pump used power to couple into the the pump ring laserpower cavity. into the A polarization-insensitivering laser cavity. A polarization- isolator polarization controllers (PCs) were employed to adjust the cavity polarization and intra-cavity (PI-ISO)insensitive was isolator employed (PI-ISO) to ensure was the employed unidirectional to en lasersure operation. the unidirectional Two polarization laser controllersoperation. (PCs)Two birefringence. A 40/60 optical coupler was used to extract a 60% lasing signal for monitoring. A piece werepolarization employed controllers to adjust the(PCs) cavity were polarization employed and to intra-cavityadjust the cavity birefringence. polarization A 40/ 60and optical intra-cavity coupler of 110 m single-mode fiber (SMF-28e) with a dispersion parameter of 17 ps/nm/km was added into wasbirefringence. used to extract A 40/60 a 60% optical lasing coupler signal was for monitoring.used to extract A piecea 60% of lasing 110 m signal single-mode for monitoring. fiber (SMF-28e) A piece the cavity for dispersion management. The total length of the cavity was approximately 120 m. Thus, withof 110 a dispersionm single-mode parameter fiber (SMF-28e) of 17 ps/nm with/km a wasdisper addedsion intoparameter the cavity of 17 for ps/nm/km dispersion was management. added into the total net cavity dispersion was calculated to be −2.6 ps2. The performance of the output laser was Thethe cavity total lengthfor dispersion of the cavity management. was approximately The total leng 120th m.of the Thus, cavity the was total approximately net cavity dispersion 120 m. Thus, was record by a fastspeed photodetector2 (3G), a digital oscilloscope (DPO4054), an optical power meter calculatedthe total net to cavity be 2.6 dispersion ps . The performance was calculated of the to be output −2.6 ps laser2. The was performance record by a fastspeedof the output photodetector laser was (BAGGER D50A),− an optical spectrum analyzer (AQ6317B) and a radio-frequency (RF) spectrum (3G),record a digitalby a fastspeed oscilloscope photodetector (DPO4054), (3G), an optical a digita powerl oscilloscope meter (BAGGER (DPO4054), D50A), an optical an optical power spectrum meter analyzer (R&S, PC1000, 1GHz). analyzer(BAGGER (AQ6317B) D50A), an and optical a radio-frequency spectrum analyzer (RF) spectrum (AQ6317B) analyzer and a (R&S, radio-frequency PC1000, 1GHz). (RF) spectrum analyzer (R&S, PC1000, 1GHz).

Figure 4. Schematic illustration of the typical all-fiber ring cavity of the mode-locked erbium-doped Figure 4. Schematic illustration of the typical all-fiber ring cavity of the mode-locked erbium-doped fiberfiber laserlaser (EDFL)(EDFL) basedbased onon thethe PVD-InPVD-In22Se33 SA. Figure 4. Schematic illustration of the typical all-fiber ring cavity of the mode-locked erbium-doped 4. Experimentalfiber laser (EDFL) Results based and on Discussionsthe PVD-In2Se3 SA.

A ~120 m long laser cavity was designed for obtaining mode-locked operations. In addition, the phenomena of self-mode-locked or Q-switched operations were always recorded within a long-length ring fiber laser cavity due to the Kerr effect under high pump power. Thus, firstly, a piece of FM substrate was inserted into the ring laser cavity instead of the In2Se3 SA for testing the possibility of self-mode-locked or Q-switched operations. By adjusting the pump power and Nanomaterials 2019, 9, 1216 6 of 12 the polarization states of PCs, neither the self-mode-locked nor Q-switched pulse generations were detected, which excluded the Kerr effect and the saturable absorption effect of the FM substrate. Then, the In2Se3-FM SA was inserted into the ring cavity, and self-started mode-locked operations were achieved when the pump power reached 313 mW. The fiber laser exhibited a high due to a large output coupling ratio and a relatively large insert loss of the SA. In our experiment, the stable mode-locked operations can be maintained with the pump power increasing from 313 to 1324 mW. As is known, the formations of different solitons were due to the balancement between the various nonlinear optical effects, the total laser gain and loss and the net dispersion value within the laser cavity. In our work, by adjusting the pump power and the states of the PCs, the different soliton operations were recorded successfully, which is discussed in detail below.

4.1. Bright Pulses Firstly, the wide-reported bright pulses were obtained in our work easily. The mode-locked characteristics of the bright pulse under the pump power of 1324 mW were depicted in Figure5. In detail, Figure5a shows the optical emission spectrum of the bright pulse, which centered at 1559.4 nm with a 3 dB spectral bandwidth of 0.305 nm. However, no Kelly sidebands were observed in the spectrum, which indicate the EDFL was not operated in the conventional soliton regime. The radio frequency (RF) spectrum was measured and shown in Figure5b. The signal-to-noise ratio (SNR) at the fundamental repetition rate of 1.71 MHz is approximately 42 dB. In order to avoid the photodetector and optical spectrum analyzer from being damaged at high output power, the output pulse was split by two output couplers with coupling rates of 40/60 and 50/50, respectively, which reduced the SNR. Accordingly, the SNR should be greater than 42 dB. Meanwhile, it also limited the wideband RF output. Figure5c shows an oscilloscope trace of the obtained single pulse. The inset shows a typical pulse train of the mode-locked operation with a pulse-to-pulse interval of 584.8 ns, corresponding to a fundamental repetition rate of 1.71 MHz, which matches well with the total cavity length of 120 m. As is shown, the full width at half-maximum (FWHM) of the bright pulse is 14.4 ns. It is generally known that the dispersion has an obvious effect on broadening the width of the pulse in an anomalous dispersion regime. Thus, the wide pulse width is mainly caused by the large net dispersion value. However, due to the limitation of response time of detector and oscilloscope, the actual pulse width will be less than 14.4 ns. Regrettably, due to the lack of autocorrelator, the actual pulse width was not measured. The average output power as a function of pump power is recorded in Figure5d. The maximum average output power is 122.4 mW at the pump power of 1324 mW. However, the single pulse energy is limited to 5.8 nJ due to the direct current power between the pulses. In our experiment, no material damage was found, which indicates that the laser damage threshold of the PVD-In2Se3 SA should be greater than 24 mJ/cm2. In addition, in further works, we hope to achieve large-energy mode-locked operations by adjusting the laser parameters and further optimizing the preparation of In2Se3 flakes. To fully prove the advantages of the PVD-In2Se3 based EDFL, the results of a series of high-power mode-locked fiber lasers based on different SAs are summarized in Table1. It is clear that the highest average output power based on mode-locked EDFL was obtained in our work due to the combination of the high pump source and the high damage-threshold SA. It is worth noting that most of the work exhibits the pulse duration of picoseconds or even sub-picoseconds, which has broad application prospects in biomedical, nonlinear optics and ultrafast optics. However, its output power is limited to tens of milliwatts [32,36,48]. When a picosecond or femtosecond pulse is amplified, the high peak power results in a strong nonlinear effect, which is not conducive to the application of high-power lasers. Compared with picosecond or femtosecond mode-locked pulses, nanosecond mode-locked pulses have advantages such as a strong chirp, large pulse width, low peak power, and small nonlinear phase shift accumulation. Therefore, the nanosecond fiber lasers with high pulse energy can be used directly as seed sources to achieve higher power output through single or multi-stage amplification Nanomaterials 2019, 9, 1216 7 of 12

systems. Our results indicate that the PVD-In2Se3-FM based mode-locked fiber laser can be used as a seedNanomaterials source 2019 for, a9, chirped1216 pulse amplification (CPA) system. 7 of 13

-5 -30

(a)-20 (b) -10 λc=1559.4nm fc=1.71MHz -40

-15 -40 -50 -20 Intensity(dBm) -60 S/N:42dB

-25 -60 -80 1558 1560 1562 1564 Intensity (dBm) Intensity (dBm) Intensity -30 Wavelength (nm) -70

-35 Δλ=0.305nm -80 -40 1559.2 1559.4 1559.6 1559.8 1560.0 1560.2 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Wavelength (nm) Frequency (MHz) 0.45 80 0.45 (c) 584.8ns 120 (d) 0.40 0.40 70 0.35 0.35 0.30 100 60 0.30 0.25 0.20 80 50

0.25 Intensity (a.u.) 0.15

0.10 40 0.20 60 0.05 30 Intensity (a.u.) Intensity 0.15 -8 -6 -4 -2 0 2 4 6 8

40 (nJ) energy Pulse Time (μs) 0.10 Average output power(mW) 20 Pulse energy(nJ)

0.05 (mW) power output Average 20 10

-100 0 100 200 300 400 200 400 600 800 1000 1200 1400 Time (ns) Pump power (mW)

Figure 5. Typical optical characteristics of the brightbright pulse at the pump power of 1324 mW. ((a)a) Optical spectrum withwith indicated 3dB3dB bandwidth. Inset: The The output output optical optical spectrum; spectrum; (b) (b) radio-frequency radio-frequency (RF) spectrum atat aa fundamental fundamental frequency frequency of 1.71of 1.71 MHz MHz with with 300 Hz300 resolution;Hz resolution; (c) the (c) corresponding the corresponding single pulsesingle profilepulse profile of the brightof the pulsebright and pulse the and inset the shows inset theshows typical the typical pulse train; pulse ( dtrain;) average (d) average output output power versuspower theversus pump the power.pump power. Table 1. Comparison of 1.5 µm high-power mode-locked fiber lasers based on different SAs a. Table 1. Comparison of 1.5 μm high-power mode-locked fiber lasers based on different SAs a.

SA Fabrication λc nm fc MHz τ ps Pave mW Epulse nJ Ref. SA Fabrication λc nm fc MHz τ ps Pave mW Epulse nJ Ref. Graphene ME 1568 16.34 0.844 30 1.84 [35] Graphene ME 1568 16.34 0.844 30 1.84 [35] Graphene RGO 1555 15.36 0.51 80 5.2 [36] Graphene RGO 1555 15.36 0.5125.16 80 5.2 [36] Bi Se SPE 1562.1 3.54 10 2.824 [37] Bi22Se33 SPE 1562.1 3.54 25.16ns ns 10 2.824 [37] BiBi22SeSe33 LPELPE 1560.5 0.5376 0.5376 — — 33.8 62.87 [[17]17] BiBi22Te33 PLDPLD 1564.1 29502950 0.920.92 45.345.3 0.01536 [[38]38] BiBi22Te33 SolvothermalSolvothermal 1571 10.71 10.71 6.2 6.2 30 2.8 [ [39]39] BiBi22Te33 MEME 1560 1.71.7 12.812.8 ns ns 32.932.9 22.4 [[40]40] SbSb22Te33 PLDPLD 1530 94 94— — 12 0.127 [[41]41] WSWS22 CVDCVD 1568.3 0.4870.487 1.491.49 62.562.5 128.3 [[42]42] WS2 LPE 1531.51531.5/1/1557.5 2.14 11 14.2 6.64 [43] WSWS2 PLDLPE 1561 2.14101.4 110.246 14.218 6.64— [[43]44] 2 557.5 WSe2 CVD 1557.4 63.133 0.1635 28.5 — [45] WS2 PLD 1561 101.4 0.246 18 — [44] WSe2 CVD 1562 58.8 0.185 30 — [46] WSe2 CVD 1557.4 63.133 0.1635 28.5 — [45] MoSe2 BTS 1557.3 3270 0.751 22.8 0.0067 [47] WSe2 CVD 1562 58.8 0.185 30 — [46] MoTe2 MSD 1559 26.6 0.229 57 2.14 [48] MoSeReS2 2 CVDBTS 1557.3 1565 3270 318.5 0.751 — 22.8 12 0.0067 0.037 [[47]49] MoTeBP 2 LPEMSD 1559.51559 26.68.77 0.2290.94 5753 2.14— [[48]22] InReS2Se23 MSDCVD 1565 318.5 40.9 — 0.276 83.212 0.037 2.03 [[49]32] In2BPSe 3 PVDLPE 1559.51559.7 8.771.71 0.9414.4 ns 122.453 5.8— This[22] work a In2λSec,3 the central wavelength;MSD f c, the fundamental1565 frequency;40.9 τ, the0.276 pulse duration;83.2 Pave, the output2.03 average power;[32] InE2pulseSe3, the pulse energy;PVD ME, mechanical1559.7 exfoliation; 1.71 RGO, reduced14.4 graphene ns oxide;122.4 SPE, solution-phase5.8 exfoliation;This work LPE, liquid-phase exfoliation; PLD, ; CVD, chemical vapor deposition; BTS, bath-type a sonication; λc, the central MSD,magnetron-sputtering wavelength; fc, the deposition.fundamental frequency; τ, the pulse duration; Pave, the output average power; Epulse, the pulse energy; ME, mechanical exfoliation; RGO, reduced graphene oxide; SPE, solution-phase exfoliation; LPE, liquid-phase exfoliation; PLD, pulsed laser deposition; CVD, chemical vapor deposition; BTS, bath-type sonication; MSD, magnetron-sputtering deposition.

To fully prove the advantages of the PVD-In2Se3 based EDFL, the results of a series of high- power mode-locked fiber lasers based on different SAs are summarized in Table 1. It is clear that the

Nanomaterials 2019, 9, 1216 8 of 12

4.2. Dark-Bright Pulse Pairs In the experiment, just by adjusting the states of the PCs, stable dark-bright pulse pair mode-locked generation has also been detected. Here, the optical characteristics of the dark-bright pulse pair under the pump power of 1324 mW were discussed. The typical output optical emission spectrum is recorded in Figure6a. It is noteworthy that the spectrum shows a typical M-shape profile with a dual-wavelength centered at 1559.4 and 1560.6 nm, respectively, which is similar to the previous report [18]. As described above, the large-area In2Se3 flakes synthesized by the PVD method exhibit a large nonlinear optical effect due to their high crystal quality and high flatness. Therefore, the high nonlinear optical effect within the laser cavity facilitates the generation of multi-wavelength mode-locked pulses. Figure6b shows the RF spectrum of the laser at a fundamental repetition rate of 1.71 MHz. The SNR exceeded 40 dB, which indicates a high stability of the dark-bright pulse pair mode-locked operation. Figure6c shows the corresponding single pulse profile of the dark-bright pulse pair. However, compared with bright pulse, the dark pulse exhibits different pulse intensity and pulse width, which is different from the previous study [50]. In our opinion, this is caused by the high-order nonlinear effect of the In2Se3 SA. Meanwhile, the inset shows the typical dark-bright pulse pair train with a period of 584.8 ns, corresponding to a fundamental repetition rate of 1.71 MHz, which verifies the mode-locked operation of the fiber laser. In addition, by adjusting the pump power and the PCs, the dark-bright pulse pair emissions also could be obtained. The relationship between the pump power and average output power is recorded in Figure6d. The mode-locked threshold of the dark-bright pulse pair operation was 397 mW, which was higher than the bright pulse mode-locked operation. The generation of dark-light pulse pair may be the result of the nonlinear refractive index of In2Se3 flakes interacting with the high nonlinear effects caused by the higher power in the laser cavity [51]. The average output power and the single pulse energy are 121.2 mW and 2.7 nJ, respectively. To the best of our knowledge, this is the highestNanomaterials output 2019,power 9, 1216 of dark-bright pulse pair mode-locked operations based on EDFL. 9 of 13

-30 -20 (a) (b) 1559.4nm 1560.6nm fc=1.71MHz -30 -40

-40 -50 S/N:40dB -50 -60

-60 Intensity (dBm) Intensity Intensity (dBm) -70

-70 -80 -80 1557 1558 1559 1560 1561 1562 1563 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Wavelength (nm) Frequency (MHz) 0.28 80 0.22 0.26 584.8ns (c) 0.20 120 (d) 0.24 0.18 70 0.16 0.22 0.14 100 60 0.20 0.12 0.10 0.18 Intensity (a.u.) 0.08 80 50 0.16 0.06 0.04 0.14 40 -8-6-4-202468 60 0.12 Time (μs) Intensity (a.u.) Intensity 0.10 30 Pulse energy (nJ)

40 0.08 Average output power(mW) 0.06 Pulse energy(nJ) 20 0.04 Average output power (mW) 20 10 -40 -20 0 20 40 60 80 100 120 140 160 180 200 400 600 800 1000 1200 1400 Time (ns) Pump power (mW)

Figure 6. TypicalTypical optical optical characteristics characteristics of of dark-brigh dark-brightt pulse pulse pairs pairs at atthe the pump pump power power of 1324 of 1324 mW. mW. (a) (Thea) The output output optical optical spectrum; spectrum; (b) (b RF) RF spectrum spectrum at at a afundamental fundamental frequency frequency of of 1.71 1.71 MHz MHz with with 300 Hz resolution; ((c)c) thethe corresponding singlesingle pulsepulse profileprofile ofof thethe dark-brightdark-bright pulsepulse pairpair andand thethe inset shows thethe pulsepulse train;train; ((d)d) averageaverage outputoutput powerpower versusversus thethe pumppump power.power.

Accordingly, the adjustment of the polarization state of the PCs also led to the formation of stable bright-dark pulse pair mode-locked operations. Figure 7 shows the output characteristics of the bright-dark pulse pair at the maximum pump power. It is obvious that the bright and dark pulses are separated from each other. Figure 7(a) shows the typical emission optical spectrum with a dual- wavelength centered at 1559.5 and 1560.7 nm, respectively. For investigating the stability of the bright-dark pulse pair mode-locked operation, its RF spectrum was measured. As shown in Figure 7(b), the fundamental frequency is also located at 1.71 MHz with a SNR over 40 dB, indicating that the bright-dark pulse pair operates in a relatively stable regime. Figure 7(c) shows the corresponding single pulse profile of the bright-dark pulse pair. Clearly, the bright and dark pulse can be observed simultaneously, which the pulse interval exceeds to approximately 200 ns. Figure 7(d) depicted the relationship between the average output power versus the pump power. As is shown, when the pump power increases from 397 to 1324 mW, the average output power grows from 32.1 to 121.5 mW, corresponding to a maximum single pulse energy of 2.8 nJ. This is the first demonstration based on In2Se3 SA in a bright-dark pulse pair mode-locked EDFL. In addition, there were no bright pulse and bright-dark pulse pairs observed no matter how adjusted the cavity polarization state was when the In2Se3 SA was removed.

Nanomaterials 2019, 9, 1216 9 of 12

Accordingly, the adjustment of the polarization state of the PCs also led to the formation of stable bright-dark pulse pair mode-locked operations. Figure7 shows the output characteristics of the bright-dark pulse pair at the maximum pump power. It is obvious that the bright and dark pulses are separated from each other. Figure7a shows the typical emission optical spectrum with a dual-wavelength centered at 1559.5 and 1560.7 nm, respectively. For investigating the stability of the bright-dark pulse pair mode-locked operation, its RF spectrum was measured. As shown in Figure7b, the fundamental frequency is also located at 1.71 MHz with a SNR over 40 dB, indicating that the bright-dark pulse pair operates in a relatively stable regime. Figure7c shows the corresponding single pulse profile of the bright-dark pulse pair. Clearly, the bright and dark pulse can be observed simultaneously, which the pulse interval exceeds to approximately 200 ns. Figure7d depicted the relationship between the average output power versus the pump power. As is shown, when the pump power increases from 397 to 1324 mW, the average output power grows from 32.1 to 121.5 mW, corresponding to a maximum single pulse energy of 2.8 nJ. This is the first demonstration based on In2Se3 SA in a bright-dark pulse pair mode-locked EDFL. In addition, there were no bright pulse and bright-dark pulse pairs observed no matter how adjusted the cavity polarization state was when the

InNanomaterials2Se3 SA was 2019 removed., 9, 1216 10 of 13

-30 -20 (a) (b) 1560.7nm 1559.5nm fc=1.71MHz -30 -40

-40 -50 S/N:40dB -50 -60

-60 Intensity (dBm) Intensity Intensity (dBm) Intensity -70

-70 -80 -80 1558 1559 1560 1561 1562 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Wavelength (nm) Frequency (MHz) 0.24 80 0.18 584.8ns (c) 120 (d) 0.22 0.16 70 0.20 0.14 0.12 100 60 0.18 0.10 Intensity (a.u.) 0.16 0.08 50 0.06 80

0.14 0.04 -8 -6 -4 -2 0 2 4 6 8 40 0.12 Time (μs) 60

Intensity (a.u.) Intensity 0.10 30 Pulse energy (nJ)

0.08 40 Average output power(mW) Pulse energy(nJ) 20 0.06 Average output power (mW) 0.04 20 10 -300 -200 -100 0 100 200 300 400 600 800 1000 1200 1400 Time (ns) Pump power (mW)

Figure 7. TypicalTypical optical optical characteristics characteristics of of bright-dar bright-darkk pulse pulse pair pair at atthe the pump pump power power of 1324 of 1324 mW. mW. (a) (Thea) The output output optical optical spectrum; spectrum; (b) (b RF) RF spectrum spectrum at at a afundamental fundamental frequency frequency of of 1.71 1.71 MHz MHz with with 300 Hz resolution;resolution; ((c)c) thethe correspondingcorresponding singlesingle pulsepulse profileprofile ofof thethe bright-darkbright-dark pulsepulse pairpair andand thethe inset shows thethe typicaltypical pulsepulse train;train; ((d)d) averageaverage outputoutput powerpower versusversus thethe pumppump power.power. 5. Conclusions 5. Conclusions In conclusion, this study has demonstrated PVD-grown large-area In2Se3 flakes as SA for In conclusion, this study has demonstrated PVD-grown large-area In2Se3 flakes as SA for generating high-power and a large-energy passively mode-locked EDFL. The In2Se3-FM SA exhibited generating high-power and a large-energy passively mode-locked2 EDFL. The In2Se3-FM SA exhibited a high laser damage threshold of higher than 24 mJ/cm 2, a large modulation depth of 18.75% and a a high laser damage threshold of2 higher than 24 mJ/cm , a large modulation depth of 18.75% and a saturable intensity of 6.8 MW/cm . Based on the In2Se3-FM SA, the stable mode-locked pulse with a saturable intensity of 6.8 MW/cm2. Based on the In2Se3-FM SA, the stable mode-locked pulse with a maximum average output power and a single pulse energy of 122.4 mW and 5.8 nJ were obtained maximum average output power and a single pulse energy of 122.4 mW and 5.8 nJ were obtained successfully. To our knowledge, this is the highest output power achieved in a mode-locked EDFL successfully. To our knowledge, this is the highest output power achieved in a mode-locked EDFL based on two-dimensional (2D) materials. In addition, the dark-bright pulse pair operations with based on two-dimensional (2D) materials. In addition, the dark-bright pulse pair operations with recorded high output powers were also observed for the first time. Thus, our experimental results recorded high output powers were also observed for the first time. Thus, our experimental results with obvious enhancements in comparison with previous works fully indicate the superiority of our experiment design and is expected to provide an absolutely new reference for generating high-power mode-locked fiber lasers based on 2D materials as SAs.

Author Contributions: Conceptualization, X.H., H.Z., and B.M.; investigation, X.H., H.Z., S.J., Q.G., and C.Z.; data curation, X.H., H.Z., and J.G.; writing—original draft preparation, X.H.; writing—review and editing, H.Z., and S.J.; funding acquisition, H.Z., B.M., C.Z., S.J., and D.L.

Funding: The authors are grateful for financial support from the National Natural Science Foundation of China (11674199, 11774208, 11804200, 61205174), Shandong Province Natural Science Foundation (ZR2017BA004, 2017GGX20120, ZR2016FP01, ZR2017BA018), Shandong Province Youth Foundation (ZR2018QF006) and the China Postdoctoral science Foundation (2016M602177).

Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Ivanenko, A.; Kobtsev, S.; Smirnov, S.; Kemmer, A. Mode-locked long fibre master oscillator with intra- cavity power management and pulse energy ＞12 μJ. Opt. Express 2016, 24, 6650–6655. 2. Liu, J.; Xu, J.; Wang, P. High Repetition-Rate Narrow Bandwidth SESAM Mode-Locked Yb-Doped Fiber Lasers. IEEE Photonics Technol. Lett. 2018, 24, 539–541.

Nanomaterials 2019, 9, 1216 10 of 12 with obvious enhancements in comparison with previous works fully indicate the superiority of our experiment design and is expected to provide an absolutely new reference for generating high-power mode-locked fiber lasers based on 2D materials as SAs.

Author Contributions: Conceptualization, X.H., H.Z., and B.M.; investigation, X.H., H.Z., S.J., Q.G., and C.Z.; data curation, X.H., H.Z., and J.G.; writing—original draft preparation, X.H.; writing—review and editing, H.Z., and S.J.; funding acquisition, H.Z., B.M., C.Z., S.J., and D.L. Funding: The authors are grateful for financial support from the National Natural Science Foundation of China (11674199, 11774208, 11804200, 61205174), Shandong Province Natural Science Foundation (ZR2017BA004, 2017GGX20120, ZR2016FP01, ZR2017BA018), Shandong Province Youth Foundation (ZR2018QF006) and the China Postdoctoral science Foundation (2016M602177). Conflicts of Interest: The authors declare no conflicts of interest.

References

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