Titanium Disulfide Based Saturable Absorber for Generating Passively Mode-Locked and Q-Switched Ultra-Fast Fiber Lasers

nanomaterials

Article Titanium Disulﬁde Based Saturable Absorber for Generating Passively Mode-Locked and Q-Switched Ultra-Fast Fiber Lasers

Xinxin Shang 1, Linguang Guo 1, Huanian Zhang 2,*, Dengwang Li 1,* and Qingyang Yue 1,* 1 Shandong Provincial Engineering and Technical Center of Light Manipulations, Shandong Provincial Key Laboratory of Optics and Photonic Devices and Shandong Key Laboratory of Medical Physics and Image Processing, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China; [email protected] (X.S.); [email protected] (L.G.) 2 School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo 255049, China * Correspondence: [email protected] (H.Z.); [email protected] (D.L.); [email protected] (Q.Y.)

Received: 10 August 2020; Accepted: 24 September 2020; Published: 26 September 2020

Abstract: In our work, passively mode-locked and Q-switched Er-doped fiber lasers (EDFLs) based on titanium disulfide (TiS2) as a saturable absorber (SA) were generated successfully. Stable mode-locked pulses centred at 1531.69 nm with the minimum pulse width of 2.36 ps were obtained. By reducing the length of the laser cavity and optimizing the cavity loss, Q-switched operation with a maximum pulse energy of 67.2 nJ and a minimum pulse duration of 2.34 µs was also obtained. Its repetition rate monotonically increased from 13.17 kHz to 48.45 kHz with about a 35 kHz tuning range. Our experiment results fully indicate that TiS2 exhibits excellent nonlinear absorption performance and significant potential in acting as ultra-fast photonics devices.

Keywords: High-damage 2D Titanium disulﬁde materials; ultra-fast optical modulation; mode-locked and Q-switched; saturable absorber

1. Introduction Due to the advantages of short pulse width, high peak power, simple structure, and excellent beam quality, pulsed fiber lasers have recently attracted significant attention because of their wide corresponding applications, such as medical treatment [1,2], fiber communications [3,4], environmental monitoring, and industry [5–7]. Passively mode-locked or Q-switched techniques are commonly-used efficient methods for generating ultra-fast pulsed lasers [8–11]. Passive techniques including nonlinear polarization rotation (NPR) [12], nonlinear amplifying loop mirror (NALM), and saturable absorber (SA) [13] are employed for demonstrating pulsed lasers operating from visible to mid-infrared optical band. Among them, SA plays an important role in demonstrating passively mode-locked and Q-switched lasers due to their unique polarization-independent properties. Previously, semiconductor saturable absorber mirrors (SESAMs), single-wall carbon nanotubes (SWCNTs), and graphene are three main fast SAs for achieving passively mode-locked and Q-switched laser operations [14–18]. Among them, SESAM exhibits repeatable absorption performance in acting as ultra-fast optical devices. However, it also possesses clear disadvantages such as high cost, narrow absorption band and low damage threshold [19,20]. SWCNT has the advantages of low cost, ultra-fast recovery time, a high damage threshold, and low saturation intensity. However, the bandwidth of SWCNT depends on its diameter, which leads to a large light scattering loss [21]. Graphene possesses a wide absorption spectrum band due to its Dirac-like electronic band structure [22,23], but its zero-bandgap property also limits its wide potential applications [24–26]. Recently, based on the development of layered

Nanomaterials 2020, 10, 1922; doi:10.3390/nano10101922 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1922 2 of 12 materials, novel nanomaterials such as transition metal dichalcogenides (TMDs), black phosphorus (BP), MXene, and topological insulators (TIs) have emerged as ultra-fast optical devices and shown excellent absorption performance [27–41]. Recently, IV–VI group TMDs, which can be expressed with the chemical formula as MX2 (M = Mo, W, Ta, V, Nb, Re, Ti, etc; X = S, Se), have gained more attention in the field of ultrafast photonics due to their high third-order optical nonlinear susceptibility and suitable layer-independent bandgap value [42–44]. Titanium disulfide (TiS2), which is a novel IV–VI semiconductor with a layered structure, has a giant tunable band gap from ~0.05 eV for bulk structure to ~2.87 eV for 1–3 layers of structure [45,46]. The structural layers are coupled by a weak Van Der Waals force, offering the possibility for exfoliation from the bulk to several thin layers. Recently, TiS2 was reported as a superior nonlinear optical limiting material [47]. Additionally, TiS2 used as SAs for generating pulsed laser operations have also been investigated preliminarily [44,48,49]. The light from 1530 nm to 1565 nm is a conventional band (C-band) that represents the regular waveband. The optical fiber exhibits the lowest loss in the C-band and has a greater advantage in long-distance transmission systems. Applied in long-distance, ultra-long-distance, and submarine optical transmission systems, the C-band becomes more important. Therefore, 1.5-micrometer mode-locked fiber lasers should have more exploration and research. The advantages of stability, economy, and high damage threshold TiS2 prove that it is an excellent saturable absorber. The main purpose of our work is to explore the deep ultrafast photonics application of TiS2. In this case, we demonstrated passively mode-locked and Q-switched Er-doped fiber lasers (EDFL) based on a TiS2-PVA film-type SA. Mode-locked laser centred at 1531.69 nm with a 3 dB bandwidth of 1.046 nm and clear Kelly sidebands was generated, which implied that the pulse width may be about 2.36 ps. Q-switched operation operated at a central wavelength of 1531.67 nm. The maximum pulse energy and minimum pulse duration were 12.7 nJ and 3.42 µs, respectively. By reducing the length of the laser cavity and optimizing the cavity loss, the maximum pulse energy and minimum pulse width were optimized to be 67.2 nJ and 2.34 µs, respectively.

2. Preparation and Characteristics of the TiS2 Material In recent work, different methods have been adapted to prepare saturable absorbers, such as optically deposition, chemical vapor deposition (CVD), and deposition in tapered fibers. A saturable absorber with a sandwich structure prepared a PVA film was applied in this experiment. The saturable absorber prepared by this method is easy to control and transfer, and the preparation method is simple and the cost is low. The film-type TiS2-polyvinyl alcohol (PVA) SA used in our experiment was prepared by the method of the liquid-phase exfoliation and spin coating. Firstly, we added 0.3 g TiS2 powder into 30 mL of 30% alcohol for preparing TiS2 solution. The solution was placed in an ultrasonic cleaner for 6 h. Few-layer TiS2 nanosheets were obtained. Then, the TiS2 solution and 4 wt% PVA solution were mixed at a volume ratio of 1:1 and placed in the ultrasonic cleaner for 6 h for preparing uniform TiS2-PVA solution. Then, 50 µL dispersion solution was spin coated on a culture dish to form TiS2-PVA film, It was then put the culture dish into a drying oven at 35 ◦C for 12 h. Lastly, a piece of the film with the size of ~1 1 mm2 was gently cut off and attached on a clean FC/PC fiber ferrule as a × proposed modulator. For testing the surface morphology of the TiS2 nanosheets, the TiS2 powder was detected by a scanning electron microscope (SEM). Figure1a depicts the SEM image of TiS 2 nanosheets, as is shown. The TiS2 nanosheets possess a clear layered structure. The energy dispersion spectroscopy (EDS) spectrum of the TiS2 nanosheets is shown in Figure1b. The corresponding peaks associated with titanium and sulfide are clearly observed. The atomic ratio of sulfur and titanium is 63:37, which corresponds to the formula of TiS2. Our results indicate that pure TiS2 nanosheets with a well layered-structure are prepared successfully. Nanomaterials 2020, 9, x FOR PEER REVIEW 3 of 12

Nanomaterials 2020, 9, x FOR PEER REVIEW 3 of 12 Nanomaterials 2020, 10, 1922 3 of 12

Figure 1. (a) SEM image of the TiS2 nanosheets. (b) EDS spectrum of the TiS2 nanosheets. Figure 1. (a) SEM image of the TiS2 nanosheets. (b) EDS spectrum of the TiS2 nanosheets. Figure 1. (a) SEM image of the TiS2 nanosheets. (b) EDS spectrum of the TiS2 nanosheets. The X-ray diffraction (XRD) of TiS2 was given in Figure 2(a) with characteristic peaks between 10° toThe 90° X-ray for characterizing diﬀraction (XRD) its ofcrystal TiS2 was structure. given inThe Figure strong2a withdiffraction characteristic peaks peakslocated between at (001), 10 (011),◦ to 90 for characterizingThe X-ray diffraction its crystal (XRD) structure. of TiS The2 was strong given diﬀ ractionin Figure peaks 2(a) located with characteristic at (001), (011), peaks (012), between and (012),◦ and (110), which is consistent with the TiS2 crystal spectrum in the standard spectrum library (110),10° which to 90° is consistentfor characterizing with the its TiS crystalcrystal structure. spectrum The in the strong standard diffraction spectrum peaks library located (PDF-65-3372). at (001), (011), (PDF-65-3372). In addition, Figure 2(b)2 shows the Raman spectrum of the TiS2 in the range of 50–400 (012), and (110), which is consistent with the TiS2 crystal spectrum in the standard1 spectrum library Incm addition,−1 using Figurethe 5322 bnm shows excitation the Raman line at spectrum room temperature. of the TiS 2 inTwo the typical range of Raman 50–400 peaks cm− atusing 230 theand 532 332 nm excitation(PDF-65-3372). line at In room addition, temperature. Figure 2(b) Two shows typical the Raman Raman peaks spectrum at 230 andof the 332 TiS cm2 in1 ,the corresponding range of 50–400 cm−1, corresponding to the Eg and A1g modes of the TiS2 are recorded, which shows− a negligible shift cm−1 using the 532 nm excitation line at room temperature. Two typical Raman peaks at 230 and 332 toin the comparison Eg and A 1gwithmodes previous of the results TiS2 are [50–52]. recorded, which shows a negligible shift in comparison with previouscm−1, resultscorresponding [50–52]. to the Eg and A1g modes of the TiS2 are recorded, which shows a negligible shift in comparison with previous results [50–52].

240 TiS2 (a) (b) 332 cm-1 0 1 1 PDF-65-3372 220 240 TiS2 (a) (b) -1 200 332 cm 0 1 1 PDF-65-3372 220

180 200

0 0 1 230 cm-1 160 180 Intensity (a.u.) Intensity 0 1 2 Intensity (a.u.) Intensity 0 0 1 230 cm-1 1 1 0 140 160 Intensity (a.u.) Intensity 0 1 2 Intensity (a.u.) Intensity

1 1 0 120 140 10 20 30 40 50 60 70 80 180 210 240 270 300 330 360 390 -1 2θ (degree) 120 Raman (cm ) 10 20 30 40 50 60 70 80 180 210 240 270 300 330 360 390 θ Raman (cm-1) Figure 2. (a) XRD2 analyze(degree) for TiS2 nanosheets. (b) Raman spectrum of TiS2 nanosheets. Figure 2. (a) XRD analyze for TiS2 nanosheets. (b) Raman spectrum of TiS2 nanosheets.

The AFM imageFigure shown2. (a) XRD in Figureanalyze3 afor was TiS2 recorded nanosheets. to determine(b) Raman spectrum the thickness of TiS of2 nanosheets. TiS 2 nanosheets The AFM image shown in Figure 3(a) was recorded to determine the thickness of TiS2 nanosheets by an atomic force microscope (AFM), and the corresponding height of the selected areas in Figure3a by an atomic force microscope (AFM), and the corresponding height of the selected areas in Figure were 12,The 13, 13,AFM and image 16 nm, shown corresponding in Figure 3(a) to was 21–29 recorded layers sinceto determine the monolayer the thickness thickness of TiS of2 nanosheets TiS2 is 3(a) were 12, 13, 13, and 16 nm, corresponding to 21–29 layers since the monolayer thickness of TiS2 0.57by nm. an As atomic sketched force in microscope Figure3c, the (AFM), transmission and the electroncorresponding microscope height (TEM) of the image selected indicates areas in that Figure is 0.57 nm. As sketched in Figure 3(c), the transmission electron microscope (TEM) image indicates the TiS3(a)2 nanosheetswere 12, 13, have 13, and a clear 16 layerednm, corresponding structure. Figure to 21–293d presents layers since the linear the monolayer transmission thickness spectrum of TiS2 that the TiS2 nanosheets have a clear layered structure. Figure 3(d) presents the linear transmission of TiSis2 0.57-PVA nm. film As on sketched the substrate, in Figure and 3(c), the substrate the transmission was also electron provided microscope to show the (TEM) linear image absorption indicates spectrum of TiS2-PVA film on the substrate, and the substrate was also provided to show the linear fromthat TiS 2thenanosheets. TiS2 nanosheets The dip have near a 400 clear nm layered in the transmission structure. Fi spectrumgure 3(d) ispresents a typical the fingerprint linear transmission of TiS2 absorption from TiS2 nanosheets. The dip near 400 nm in the transmission spectrum is a typical nanosheets.spectrum The of absorptionTiS2-PVA film band on extends the substrate, from 400 and nm the to 2000subs nm.trate Thewas nonlinear also provided absorption to show property the linear fingerprint of TiS2 nanosheets. The absorption band extends from 400 nm to 2000 nm. The nonlinear of theabsorption TiS2-PVA from film wasTiS2 investigatednanosheets. The by a dip power-dependent near 400 nm in transmission the transmission technique, spectrum as shown is a typical in absorption property of the TiS2-PVA film was investigated by a power-dependent transmission Figurefingerprint3e. The relationship of TiS2 nanosheets. between The the absorption transmittance band and extends the optical from 400 intensity nm to can2000 be nm. fitted The by nonlinear the technique, as shown in Figure 3(e). The relationship between the transmittance and the optical functionabsorption shown property in the following of the Equation:TiS2-PVA film was investigated by a power-dependent transmission intensity can be fitted by the function shown in the following Equation: technique, as shown in Figure 3(e). The relationship between the transmittance and the optical intensity can be fitted by the functionT(I) = 1shown∆T. expin the( Ifollowing/Isat) Tns Equation: T (I) −= 1− ΔT.exp(− −I /−I ) −T sat ns where T and Tns denotes transmission and non-saturable= − Δ loss,− and ∆T is− modulation depth, I and Isat T (I) 1 T.exp( I / I sat ) Tns are input intensity of laser and saturation intensity. The modulation depth and saturation intensity is

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Nanomaterialswhere T and2020 ,T10ns, 1922denotes transmission and non-saturable loss, and ΔT is modulation depth, I 4and of 12 Isat are input intensity of laser and saturation intensity. The modulation depth and saturation intensity is calculated to be 13.19% and 17.97 MW/cm2, respectively. The measured results indicate that TiS2 is 2 calculateda promising to be broadband 13.19% and optical 17.97 material MW/cm for, respectively. ultrashort pulse The measuredgeneration results in lasers. indicate that TiS2 is a promising broadband optical material for ultrashort pulse generation in lasers.

Figure 3. (a) AFM image of TiS2 nanosheets. (b) Corresponding height profile of the selected area in Figure 3. (a) AFM image of TiS2 nanosheets. (b) Corresponding height profile of the selected area in (a). (c) TEM image of TiS2 nanosheets. (d) Linear transmission of the TiS2-PVA film SA. (e) Nonlinear (a). (c) TEM image of TiS2 nanosheets. (d) Linear transmission of the TiS2-PVA film SA. (e) Nonlinear absorption property of the TiS2-PVA film. absorption property of the TiS2-PVA film. The saturable absorption theory can be explained by the Pauli blocking principle as a typical The saturable absorption theory can be explained by the Pauli blocking principle as a typical 2D 2D material. Linear absorption can occur under low excitation intensity. The electrons distributed in material. Linear absorption can occur under low excitation intensity. The electrons distributed in the the valence band can absorb the energy of the incident light and be excited into the conduction band valence band can absorb the energy of the incident light and be excited into the conduction band when the energy of the incident light is greater than the bandgap value of the TiS2. Afterward, the hot when the energy of the incident light is greater than the bandgap value of the TiS2. Afterward, the electrons cool down almost immediately and cause the formation of a hot Fermi-Dirac distribution. hot electrons cool down almost immediately and cause the formation of a hot Fermi-Dirac In this case, the newly generated electron-hole pairs will prevent the initial potential inter-band distribution. In this case, the newly generated electron-hole pairs will prevent the initial potential optical transitions and photons absorption around the Fermi energy ( E/2). Lastly, due to the internal inter-band optical transitions and photons absorption around the Fermi− energy (−E/2). Lastly, due to scattering of phonons, electrons and holes recombine and reach a state of equilibrium distribution. the internal scattering of phonons, electrons and holes recombine and reach a state of equilibrium However, the concentration of photocarriers will increase instantaneously and the energy states near distribution. However, the concentration of photocarriers will increase instantaneously and the the edges of the conduction band and valence band will be filled under higher excitation intensity. energy states near the edges of the conduction band and valence band will be filled under higher Since no two electrons can reach the same state defined by the Pauli blocking principle, the absorption excitation intensity. Since no two electrons can reach the same state defined by the Pauli blocking will be blocked. Therefore, specific frequency photons transmit the material without absorption. As is described, the bandgap value has a great significance for the design of ultra-fast photonics devices. Nanomaterials 2020, 9, x FOR PEER REVIEW 5 of 12

principle, the absorption will be blocked. Therefore, specific frequency photons transmit the material without absorption. As is described, the bandgap value has a great significance for the design of ultrafast photonics devices. Nanomaterials 2020, 10, 1922 5 of 12 3. Experimental Setup

3. ExperimentalFor testing the Setup nonlinear absorption properties of the TiS2 SA, an all-fiber ring laser cavity was demonstrated and shown in Figure 4. A 28-cm long Er-doped fiber (LIEKKI Er80-8/125, nlight, For testing the nonlinear absorption properties of the TiS2 SA, an all-fiber ring laser cavity was 2 −1 demonstratedVancouver, WA, and USA) shown with in a Figuregroup4 velocity. A 28-cm dispersion long Er-doped of −22.6 ps fiberkm (LIEKKI was used Er80-8 as the/125, laser nlight, gain Vancouver,medium and WA, pumped USA) with by a a 980-nm group velocity laser diode dispersion (LD) with of 22.6 the psmaximum2km 1 was pump used power as the laserof 1.3 gain W − − mediumthrough anda 980/1550 pumped nm by wavelength a 980-nm laser division diode (LD)multiplexer with the (WDM). maximum Another pump powerport of of the 1.3 WDM W through was aattached 980/1550 to nmthe wavelength90% port of divisionan optical multiplexer coupler (O (WDM).C). A polarization-insensitive Another port of the WDM optical was isolator attached (PI- toISO) the was 90% employed port of an to optical ensure coupler the unidirectional (OC). A polarization-insensitive operation and two polarization optical isolator controllers (PI-ISO) (PCs) was employedwere used toto ensureadjust the unidirectionalpolarization state. operation In addition, and two a 50-m polarization long single-mode controllers fiber (PCs) (SMF) were with used a todispersion adjust the parameter polarization of ~17 state. ps/(nm In addition, × km) was a 50-m adde longd into single-mode the cavity fiber to adjust (SMF) the with net a dispersion. dispersion parameterFurthermore, of ~1710% ps output/(nm characteristicskm) was added were into analyzed the cavity by toa 3-GHz adjust photo-detector the net dispersion. (PD 03) Furthermore, combined × 10%a digital output oscilloscope characteristics (Wavesurfer were analyzed 3054, Teledyne by a 3-GHz LeCroy, photo-detector USA), a power (PD 03)meter combined (PM100D-S122C a digital, oscilloscopeThorlabs, New (Wavesurfer Jersey, American 3054, Teledyne),a radio-frequency LeCroy, USA), spectrum a power (R&S meter FPC1000, (PM100D-S122C, Jena Germany), Thorlabs, and Newan optical Jersey, spectrum American),a analyzer radio-frequency (AQ6317B, Yokogawa spectrum, Tokyo, (R&S FPC1000, Japan). Jena Germany), and an optical spectrum analyzer (AQ6317B, Yokogawa, Tokyo, Japan).

Figure 4. Schematic diagram of the experimental setup.

4. Results and DiscussionFigure 4. Schematic diagram of the experimental setup.

4.1.4. Results Mode-Locked and Discussion Operation Optimization before carrying out the experiment, the output characteristics of the ﬁber laser 4.1. Mode-Locked Operation without inserting the TiS2 modulator into the ring laser cavity was tested. No mode-locked or Q-switchedOptimization pulses werebefore recorded carrying by out adjusting the experiment, the pump the power output and polarizationcharacteristics states of the of PCs.fiber Then,laser thewithout TiS2 modulatorinserting the was TiS inserted2 modulator into into the laserthe ring cavity, laser as cavity sketched was intested. Figure No4. mode-locked When the pump or Q- powerswitched increased pulses towere 276 recorded mW, stable by mode-lockedadjusting the pulses pump werepower observed and polarization by adjusting states the of PCs PCs. carefully. Then, Thethe TiS output2 modulator characteristics was inserted are shown into in the Figure laser5 cavity,. The output as sketched power in is Figure 0.177 mW, 4. When which the corresponds pump power to aincreased single pulse to 276 energy mW, of stable 0.05 nJ. mode-locked As shown in pulses Figure were5a, a typicalobserved soliton by adjusting spectrum the with PCs clear carefully. symmetric The Kellyoutput sidebands characteristics is obtained, are shown which in indicates Figure 5. that The the output laser operatespower is at0.177 a traditional mW, which soliton corresponds mode-locked to a state.single Due pulse to lowenergy output of 0.05 power, nJ. it As is dishownﬃcult forin usFigure to measure 5(a), a thetypical real characteristicssoliton spectrum of pulse with width. clear Thesymmetric central Kelly wavelength sidebands located is obtained, at 1531.69 which nm withindicate halfs wavelengththat the laser full operates width at (FWHM) a traditional of 1.046 soliton nm, whichmode-locked implied state. that Due the pulseto low width output of power, the laser it is is d aboutifficult 2.36for us ps to based measure on the thesoliton real characteristics theory [53]. Figureof pulse5b depictswidth. theThe pulse central train wavelength on oscilloscope. located The at interval 1531.69 of nm pulse with train half is 291.3wavelength ns, which full matches width the(FWHM) round-trip of 1.046 time nm, of which the cavity. implied Figure that5 cthe presents pulse width the radio of the frequency laser is about (RF) spectrum2.36 ps based located on the at thesoliton fundamental theory [53]. repetition Figure 5(b) rate depicts of 3.43 the MHz pulse with train a signal-to-noise on oscilloscope. ratio The of interval ~60 dB, of corresponding pulse train is to the cavity length of 59.8 m. The higher harmonics within a wide bandwidth of 34 MHz is shown in Figure5d for investigating the long-term stability. The interval of the repetition rate is 3.43 MHz. The RF results indicate that stable mode-locked pulse operation is achieved. Nanomaterials 2020, 9, x FOR PEER REVIEW 6 of 12

291.3 ns, which matches the round-trip time of the cavity. Figure 5(c) presents the radio frequency (RF) spectrum located at the fundamental repetition rate of 3.43 MHz with a signal-to-noise ratio of ~60 dB, corresponding to the cavity length of 59.8 m. The higher harmonics within a wide bandwidth of 34 MHz is shown in Figure 5(d) for investigating the long-term stability. The interval of the Nanomaterialsrepetition2020 rate, 10 is, 19223.4 MHz. The RF results indicate that stable mode-locked pulse operation is achieved.6 of 12

-10 1.4

-20 (a) Central Wavelength 1.2 (b) 291.3 ns @1531.69 nm -30 1.0

-40 0.8 -50 1.046 nm 0.6 0.4 Intensity (dB) Intensity

-60 (a.u.) Intensity

0.2 -70

0.0 -80 1518 1521 1524 1527 1530 1533 1536 1539 1542 1545 -2 -1 0 1 2 Wavelength (nm) Time (μs)

-20 -10

-30 (c) -20 (d) 3.43 MHz -30 -40 -40 -50 -50 Central Frequency -60 -60 3.43 MHz 60 dB -70 -70

Intensity (dB) Intensity (dB) Intensity -80 -80 -90 -90 -100 -100 -110 123456 0.0 3.4 6.8 10.2 13.6 17.0 20.4 23.8 27.2 30.6 34.0 Frequency (MHz) Frequency (MHz)

Figure 5. Typical mode-locked pulse characteristics at the pump power of 276 mW. (a) The output opticalFigure spectrum 5. Typical with mode-locked clear Kelly sidebands.pulse characterist (b) Theics corresponding at the pump mode-lockedpower of 276 pulse mW. train.(a) The (c )output The fundamentaloptical spectrum repetition with rate. clear (d Kelly) The sidebands. higher harmonics (b) The withincorresponding a bandwidth mode-locked of 34 MHz. pulse train. (c) The fundamental repetition rate. (d) The higher harmonics within a bandwidth of 34 MHz. 4.2. Q-Switched Operation 4.2. Q-Switched Operation Then, for obtaining large-energy Q-switched operating commonly obtained within short-length laser cavity,Then, the for length obtaining of the large-energy ring laser cavity Q-switched was adjusted operating by cutting commonly off 30 obtained m SMF. Stablewithin self-started short-length Q-switchedlaser cavity, operation the length was of observed the ring underlaser cavity the pump was adjusted power of by 221 cutting mW. Theoff recorded30 m SMF. pulse Stable train self- withstarted good Q-switched stability is shownoperation in Figure was observed6a. The pulse-to-pulse under the pump time power is 60.24 of 221µs, whichmW. The corresponds recorded topulse a pulsetrain repetition with good rate stability of 16.6 kHz is shown (the RF in spectrum Figure is6(a). shown The in pulse-to-pulse Figure6d). The time single is pulse60.24 profileµs, which is presentedcorresponds in Figure to a6 b.pulse The repetition FWHM pulse rate duration of 16.6 kHz is about (the 7.27 RF spectrumµs. The emission is shown spectrum in Figure is provided 6(d)). The insingle Figure 6pulsec. The profile central is wavelength presented in is aboutFigure 1531.67 6(b). The nm. FWHM pulse duration is about 7.27 µs. The emissionThe repetition spectrum rate is andprovided pulse in duration Figure characteristics6(c). The central under wavelength different is pump about powers 1531.67 are nm. described in Figure7a. As sketched in Figure7a, the pulse duration reduces from 7.27 to 3.42 µs and the repetition rate increases from 16.6 to 26.7 kHz with the increase of the pump power. Figure7b describes the output power and pulse energy as a function of pump power. The maximum pulse energy is 12.7 nJ under the pump power of 407 mW. The pulse energy and pump power have a linear relationship under the pump power of 400 mW, but the pulse energy was limited when the pump power exceeded 400 mW. The measured minimum pulse width was 3.42 µs at a pump power of 374 mW. It is considered that the pulse duration could be shortened further by reducing the length of the laser cavity [54] and by optimizing the cavity loss [55,56].

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0.016 0.014

0.014 (a) 60.24 μs 0.012 (b)

0.012 0.010 0.010 0.008 0.008 7.27 μs 0.006 0.006 0.004 0.004 Intensity (a.u.) Intensity (a.u.) 0.002 0.002

0.000 0.000

-0.002 -0.002 400 500 600 700 800 900 1000 1100 1200 500 510 520 530 540 550 560 570 580 Time (μs) Time (μs) -20 -40 Nanomaterials 2020, 9, x FOR PEER REVIEW 7 of 12 Nanomaterials 2020, 10(c), 1922 (d) 7 of 12 -30 -45 -40

0.016 0.014 -50 -50 16.6 kHz 0.014 (a) 60.24 μs 0.012 (b)

0.012 -60 0.010 Intensity (dB) Intensity (dB) Intensity 0.010 -55 -70 0.008 0.008 7.27 μs 0.006 -80 0.006 -60 1524 1527 1530 1533 1536 1539 80.004 1012141618202224 0.004 Intensity (a.u.) Intensity (a.u.) Wavelength (nm) 0.002 Frequency (kHz) 0.002 0.000 0.000

-0.002 -0.002 Figure 6. The Typical400 500 Q-switched 600 700 800 pulse 900 1000characteri 1100 1200stics at 500the pump 510 520 power 530 540 of 550221 560 mW. 570 (a 580) Output pulse train at 16.6 kHz repetitionTime rate. (μs) (b) Single pulse profile with 7.27 µsTime duration. (μs) (c) The emission spectrum. (d) The-20 RF spectrum located at 16.6 kHz. -40 (c) (d) -30 -45 The repetition-40 rate and pulse duration characteristics under different pump powers are described in Figure 7(a). As sketched in Figure 7(a), the pulse duration reduces from 7.27 to 3.42 µs -50 -50 16.6 kHz and the repetition rate increases from 16.6 to 26.7 kHz with the increase of the pump power. Figure -60 7(b) describes the (dB) Intensity output power and pulse energy as a function (dB) Intensity -55 of pump power. The maximum pulse energy is 12.7 nJ under-70 the pump power of 407 mW. The pulse energy and pump power have a linear relationship under -80the pump power of 400 mW, but the-60 pulse energy was limited when the pump 1524 1527 1530 1533 1536 1539 8 1012141618202224 power exceeded 400 mW. TheWavelength measured (nm) minimum pulse width wasFrequency 3.42 µs (kHz) at a pump power of 374 mW. It is considered that the pulse duration could be shortened further by reducing the length of the Figure 6. The Typical Q-switched pulse characteristics at the pump power of 221 mW. (a) Output laser cavity [54] and by optimizing the cavity loss [55,56]. pulseFigure train 6. at The 16.6 Typical kHz repetition Q-switched rate. pulse (b) Single characteri pulsestics proﬁle at the with pump 7.27 µ powers duration. of 221 (c )mW. The emission(a) Output spectrum.pulse train (d) at The 16.6 RF kHz spectrum repetition located rate. at (b 16.6) Single kHz. pulse profile with 7.27 µs duration. (c) The emission spectrum. (d) The RF spectrum located at 16.6 kHz. 9 Pulse width 0.35 Repetition rate 30 14 8 (a) (b) The repetition rate and pulse duration characteristics0.30 under different pump powers are

s) 27

μ 12 described7 in Figure 7(a). As sketched in Figure 7(a),0.25 the pulse duration reduces from 7.27 to 3.42 µs 24 and the repetition rate increases from 16.6 to 26.7 kHz0.20 with the increase of the pump power. Figure 10 7(b) describes6 the output power and pulse energy as a function of pump power. The maximum pulse 21 0.15 5 energy is 12.7 nJ under the pump power of 407 mW. The pulse energy and pump power have8 a linear 18 0.10

Pulse width ( Output Power Pulse Pulse energy (nJ) relationship4 under the pump power of 400 mW, but the pulse energy Pulse energywas limited when the pump Output power (mW)

Repetition rate (kHz) rate Repetition 0.05 power exceeded 400 mW. The measured minimum15 pulse width was 3.42 µs at a pump power6 of 374 3 0.00 mW. 150It is 200considered 250 300 350that 400 the 450 pulse 500 duration 550 600 could be150 shortened 200 250 further 300 350 by 400 re 450ducing 500 550 the 600 length of the Pump power (mW) laser cavity [54] and by optimizing the cavity loss [55,56]. Pump power (mW) Figure 7. (a) Variation of repetition rate and pulse duration with pump power. (b) Output power and Figurepulse 7. energy (a) Variation as a function of repetition of pump rate power and pulse in the duration Q-switched with ﬁber pump laser. power. (b) Output power and 9 pulse energy as a function of pump power Pulse widthin the Q-switched0.35 fiber laser. Repetition rate 30 14 For getting8 (a) the shorter pulse duration and the more stable (b) ﬁber laser, we continued to adjust the 0.30

length of cavitys) to 7.9 m by cutting oﬀ the SMF. Stable27 self-started Q-switched operation was achieved μ 12 7 0.25 at a threshold pump power of 79 mW. Figure824a displays the Q-switched pulse train recorded by 0.20 the oscilloscope.6 The interval of pulse train is 20.9 µs, which corresponds to the repetition rate10 of 21 0.15 48.8 kHz (the5 yellow peak in Figure8c). The corresponding minimum pulse width is 2.3 µs (shown in 8 18 0.10

Pulse width ( Output Power Figure8b) at the pump power of 352 mW. The repetition rate under di ﬀerent pump power is presented Pulse energy (nJ) 4 Pulse energy Output power (mW)

Repetition rate (kHz) rate Repetition 0.05 in Figure8c. All of the repetition rates match well15 with the Q-switched pulse-to-pulse time and have6 3 0.00 high signal-to-noise150 200 250 ratios. 300 350 The 400 experimental 450 500 550 600 results exhibit150 that 200 high 250 300 stable 350Q-switched 400 450 500 550 pulses 600 are obtained by reducingPump the lengthpower (mW) of the laser cavity to optimize the cavity loss. The corresponding Pump power (mW) optical spectrum under different pump power was provided in Figure8d. The center optical spectrum was located at 1556 nm when the pump power was within 300 mW. Figure 7. (a) Variation of repetition rate and pulse duration with pump power. (b) Output power and Figurepulse9 aenergy shows as a the function pulse of width pump andpower the in the repetition Q-switched rate fiber as alaser. function of the pump power. By increasing the pump power from 79 mW to 352 mW, the repetition rate monotonically increases from 13.17 to 48.45 kHz with about a 35-kHz tuning range. The pulse width reduced from 8.49 to 2.34 µs with increasing pump power. This phenomenon is due to the gain compression in the Q-switched fiber laser [55]. Figure9b presents the average output power and the single pulse energy under di fferent

Nanomaterials 2020, 9, x FOR PEER REVIEW 8 of 12

For getting the shorter pulse duration and the more stable fiber laser, we continued to adjust the length of cavity to 7.9 m by cutting off the SMF. Stable self-started Q-switched operation was achieved at a threshold pump power of 79 mW. Figure 8(a) displays the Q-switched pulse train recorded by the oscilloscope. The interval of pulse train is 20.9 µs, which corresponds to the repetition rate of 48.8 kHz (the yellow peak in Figure 8(c)). The corresponding minimum pulse width is 2.3 µs (shown in Figure 8(b)) at the pump power of 352 mW. The repetition rate under different pump power is presented in Figure 8(c). All of the repetition rates match well with the Q-switched pulse-to-pulse Nanomaterialstime and 2020have, 10 ,high 1922 signal-to-noise ratios. The experimental results exhibit that high stable8 of 12Q- switched pulses are obtained by reducing the length of the laser cavity to optimize the cavity loss. The corresponding optical spectrum under different pump power was provided in Figure 8(d). The pump powers. The maximum average output power and single pulse energy are 3.26 mW and 67.24 nJ, center optical spectrum was located at 1556 nm when the pump power was within 300 mW. respectively. All the results prove that the cavity loss is reduced by shortening the length of the cavity.

1.0 1.0 (a) 20.9 μs (b) 0.8 0.8

0.6 0.6 2.3 μs 0.4 0.4

Intensity (a.u.) Intensity 0.2 (a.u.) Intensity 0.2

0.0 0.0

-120 -80 -40 0 40 80 120 -15 -10 -5 0 5 10 15 Time (μs) Time (μs) -20 0 79 (mW) -30 133 (mW) -40 -10 (c) 166 (mW) (d) -50 221 (mW) -60 -20 287 (mW) 352 (mW) -70 -30 -80

-90 (dB) Intensity -40 -100

287 )

-50 W

221 (

r Intensity (dB) Intensity

-60 166 w

-70 133 p

79 P -80 0 6 12 18 24 30 36 42 48 54 60 1535 1540 1545 1550 1555 1560 1565 1570 1575 Frequency (kHz) Wavelength (nm)

Figure 8. Typical Q-switched pulse characteristics with a 7.9-m long cavity. (a) Output pulse train at a Nanomaterials48.8-kHzFigure 2020 8. repetition, Typical9, x FOR Q-switchedPEER rate. REVIEW (b) Single pulse pulse characteristics proﬁle with with a 2.3- a 7.9-mµs duration long cavity. (c) and (a ()d Output) the repetition pulse train rate9 at of 12 undera 48.8-kHz diﬀerent repetition pump powersrate. (b) and Single its correspondingpulse profile with emission a 2.3-µs spectrum. duration (c) and (d) the repetition rate under different pump powers and its corresponding emission spectrum. 10 3.5 50 70 9 (a)3.0 (b) Figure. 9(a) shows Pulse the width pulse width and45 the repetition rate as a function of the pump power. By Output power 8 Repetition rate s) Pulse energy 60 2.5 increasingμ the pump power from 79 mW to40 352 mW, the repetition rate monotonically increases from 7 13.17 to 48.45 kHz with about a 35-kHz tuning35 range.2.0 The pulse width reduced from 8.4950 to 2.34 µs 6 30 with increasing5 pump power. This phenomenon is due1.5 to the gain compression in the Q-switched40 25 fiber laser4 [55]. Figure 9(b) presents the average output1.0 power and the single pulse energy under 30

Pulse width ( 20 3 Pulse (nJ) energy

different pump powers. The maximum average outpOutput power (mW) ut0.5 power and single pulse energy are 3.26 mW 15 (kHz) rate Repetition 2 20 and 67.24 nJ, respectively. All the results prove10 that the0.0 cavity loss is reduced by shortening the length of the cavity.50 100 150 200 250 300 350 50 100 150 200 250 300 350 Pump power (mW) Pump power (mW)

Figure 9. a b Figure 9. (a() )Pulse Pulse width width and and repetition repetition rate rate versus versus pump pump power. power. (b ( ) )Output Output power power variation variation and and pulse energy as a function of pump power in the Q-switched ﬁber laser. pulse energy as a function of pump power in the Q-switched fiber laser.

In 2018, Zhu et al. reported the TiS2 used as SA for the first time, which is of great significance [44]. In 2018, Zhu et al. reported the TiS2 used as SA for the first time, which is of great significance As shown in Table1, we compared the parameters of the laser obtained by our experiment with the [44]. As shown in Table 1, we compared the parameters of the laser obtained by our experiment with parameters of their laser. They used the optically deposition method to prepare SA while the sandwich the parameters of their laser. They used the optically deposition method to prepare SA while the structure was adopted in our experiment. The optical modulation depth of our SA was 13.19% higher sandwich structure was adopted in our experiment. The optical modulation depth of our SA was than 8.3% in their experiment. In the mode-locked and Q-switched pulse operation, although the center 13.19% higher than 8.3% in their experiment. In the mode-locked and Q-switched pulse operation, wavelength obtained in this experiment has a blue shift for the same nanomaterial, this is our next although the center wavelength obtained in this experiment has a blue shift for the same nanomaterial, step to explore the influence factors of the blue shift of the laser wavelength. They can get the pulse this is our next step to explore the influence factors of the blue shift of the laser wavelength. They can duration of 812 fs in a mode-locked pulse, and 2.36 ps pulse was obtained in our work. Higher single get the pulse duration of 812 fs in a mode-locked pulse, and 2.36 ps pulse was obtained in our work. pulse energy was obtained in both a mode-locked and Q-switched operation than by Zhu, X. et al. [44]. Higher single pulse energy was obtained in both a mode-locked and Q-switched operation than by The highest single pulse energy can reach 67.24 nJ. This shows that the fiber laser we designed has Zhu, X. et al [44]. The highest single pulse energy can reach 67.24 nJ. This shows that the fiber laser excellent characteristics. we designed has excellent characteristics.

Table 1. A comparison of the characters of our fiber laser with [44]. Fundamental Output SA αs Isat Wavelength Pulse Pulse Pulse Type Frequency Power Ref. Fabrication (%) (MW/cm2) (nm) Duration Energy (MHz) (mW) Mode-locked 1563.3 812 fs 22.7 25.3 pJ 0.574 optically 8.3 - 25.2 to 50.7 [44] Q-switched deposition 1560.2 4 µs 9.5 nJ 0.48 kHz Mode-locked 1531.69 2.36 ps 3.43 0.05 nJ 0.177 sandwich 13.1 Our 17.97 13.17 to 67.24 Q-switched structure 9 1556 2.34 µs 3.26 work 48.45 kHz nJ

αs, modulation depth; Isat, saturable intensity;

5. Conclusions

To sum up, TiS2 nanosheets were prepared by the liquid-phase exfoliation method and its nonlinear optical properties were investigated experimentally. Based on TiS2-PVA film-type SA, both mode-locked and Q-switched Er-doped fiber lasers were achieved. Stable mode-locked pulse centred at 1531.69 nm with the pulse width of 2.36 ps was obtained. By reducing the length of the laser cavity and optimizing the cavity loss, self-started Q-switched operation with a maximum pulse energy and a minimum pulse width of 67.2 nJ and 2.34 µs was obtained. Our findings prove that TiS2 exhibits excellent absorption performance and will have wide applications in proposing ultrafast photonic devices.

Author Contributions: X.-X.S. and H.-N.Z. conceived and designed the experiments. L.-G.G. fabricated and tested the ultra-fast modulator. D.-W.L. analyzed the experimental data. X.-X.S., H.-N.Z. and Q.-Y.Y. wrote, reviewed, and edited the paper. All authors have read and agreed to the published version of the manuscript.

Funding: The National Natural Science Foundation of China (Grant nos. 11904213, 11747149, 12074225), Shandong Province Natural Science Foundation (ZR2018QF006,ZR2019MF029), and China Postdoctoral Science

Nanomaterials 2020, 10, 1922 9 of 12

Table 1. A comparison of the characters of our ﬁber laser with [44].

SA I Wavelength Pulse Fundamental Output Pulse Type α (%) sat Pulse Energy Ref. Fabrication s (MW/cm2) (nm) Duration Frequency (MHz) Power (mW) Mode-locked optically 1563.3 812 fs 22.7 25.3 pJ 0.574 8.3 - [44] deposition Q-switched 1560.2 4 µs 25.2 to 50.7 kHz 9.5 nJ 0.48 Mode-locked sandwich 1531.69 2.36 ps 3.43 0.05 nJ 0.177 13.19 17.97 Our work Q-switchedstructure 1556 2.34 µs 13.17 to 48.45 kHz 67.24 nJ 3.26

αs, modulation depth; Isat, saturable intensity; Nanomaterials 2020, 10, 1922 10 of 12

5. Conclusions

To sum up, TiS2 nanosheets were prepared by the liquid-phase exfoliation method and its nonlinear optical properties were investigated experimentally. Based on TiS2-PVA film-type SA, both mode-locked and Q-switched Er-doped fiber lasers were achieved. Stable mode-locked pulse centred at 1531.69 nm with the pulse width of 2.36 ps was obtained. By reducing the length of the laser cavity and optimizing the cavity loss, self-started Q-switched operation with a maximum pulse energy and a minimum pulse width of 67.2 nJ and 2.34 µs was obtained. Our findings prove that TiS2 exhibits excellent absorption performance and will have wide applications in proposing ultrafast photonic devices.

Author Contributions: X.S. and H.Z. conceived and designed the experiments. L.G. fabricated and tested the ultra-fast modulator. D.L. analyzed the experimental data. X.S., H.Z. and Q.Y. wrote, reviewed, and edited the paper. All authors have read and agreed to the published version of the manuscript. Funding: The National Natural Science Foundation of China (Grant nos. 11904213, 11747149, 12074225), Shandong Province Natural Science Foundation (ZR2018QF006,ZR2019MF029), and China Postdoctoral Science Foundation (2016M602177) funded this research. The “Opening Foundation of Shandong Provincial Key Laboratory of Laser Technology and Application” also supported this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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