Broadband and enhanced nonlinear optical response of MoS2/graphene nanocomposites for ultrafast photonics applications

Due to their relatively high compatibility with specific photonic structures, strong light-matter interactions and unique nonlinear optical response, two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are attractive for ultrafast photonics applications. Here, we fabricate MoS2/graphene nanocomposites by a typical hydrothermal method. In addition, we systematically investigate their nonlinear optical responses. Our experiments indicate that the combined advantages of ultrafast relaxation, a broadband response from graphene, and the strong light-matter interaction from MoS2, can be integrated together by composition. The optical properties in terms of carrier relaxation dynamics, saturation intensity and modulation depth suggest great potential for the MoS2/graphene nanocomposites in photonics applications. We have further fabricated 2D nanocomposites based optical saturable absorbers and integrated them into a 1.5 μm Erbium-doped fiber laser to demonstrate Q-switched and mode-locked pulse generation. The fabrication of 2D nanocomposites assembled from different types of 2D materials, via this simple and scalable growth approach, paves the way for the formation and tuning of new 2D materials with desirable photonic properties and applications.

Two-dimensional (2D) materials have received increasing attention in recent years because of their unique optoelectronic properties, which are a natural consequence of the quantum confinement effect associated with their ultra-thin 2D structure [1][2][3] . Graphene, a layer of 2D sp 2 -bonded carbon atoms, has been widely researched for optoelectronic applications due to its high carrier mobility 4,5 , broad absorption spectrum 6 , remarkable nonlinear optical (NLO) properties 6,7 and ultrafast carrier dynamics [8][9][10][11] . Enlightened by the substantial advantages of graphene for optoelectronics, researchers have started to explore the graphene analogues of layered inorganic materials. Another type of 2D materials are transition metal dichalcogenides (TMDCs), such as MoS 2 , MoSe 2 , WS 2 and WSe 2 , consisting of a hexagonal layer of metal atoms (M) sandwiched between two layers of chalcogen atoms (X) within the stoichiometry MX 2 12 . Their common characteristic is a layered structure with strong intra-layer covalent bonding and weak interlayer Van der Waals forces between the MX 2 sheets. Single-and few-layer 2D TMDCs materials have been successfully fabricated through different approaches, such as mechanical or liquid 1 exfoliation [12][13][14][15] . In terms of their applications, TMDCs have already exhibited considerable potentials in various fields including field-effect transistors [16][17][18][19] , photodetectors 20 , memories 21 , integrated circuits 22,23 , passive mode lockers 24,25 and optical switchers 26 . Among them, molybdenum disulphide (MoS 2 ) has been intensively studied. Unlike the semi-metallic graphene, MoS 2 is a semiconductor with a band-gap that is dependent on the number of layers and varies from 1.29 eV 1 (bulk) to 1.9 eV (single layer) 27,28 . The band structure of MoS 2 has an indirect-to-direct transition when the thickness is reduced from bulk to monolayer 1,28 , resulting in the dramatic enhancement of photoluminescence 28 . Otherwise, MoS 2 shows strong light-matter interaction because of Van Hove singularities in the density of states 29 . The exotic optical-electrical properties make MoS 2 a potential optical material for use in optoelectronics. Most recently, the fabrication of 2D heterostructures or nanocomposites assembled by graphene and other graphene-like 2D crystals (such as MoS 2 ) has been demonstrated as a useful strategy for the realization of novel 2D materials with unique electronic and optoelectronic applications 30,31 . The strongest motivation to investigate the nanocomposites is to determine their multiple functionalities. Therefore, to further enhance the optical properties of graphene as well as those of the newly introduced functional materials, it is necessary to investigate the possibility of combining graphene and other 2D materials by specific methods to create 2D nanocomposites. Recently, MoS 2 /graphene nanocomposites were successfully synthesized 32,33 , and they show good electron conductivity in electrochemical applications. Interestingly, MoS 2 /graphene nanocomposites have also been reported for various applications of dye-sensitive solar cell fabrication 34 , reversible lithium storage 32,35 , novel biosensors and electronic devices 36 . MoS 2 /graphene nanocomposites may have similar properties and comparable performance as materials containing composites 37 . Although experimentalists have shown great interests in other properties of MoS 2 /graphene nanocomposites, their ultrafast carrier dynamics and nonlinear optical properties remain un-explored. However, understanding the ultrafast and nonlinear optical properties is critical for the use of MoS 2 / graphene nanocomposites in ultrafast photonics applications. Herein, we have investigated the carrier dynamics of the photo-excited states of MoS 2 /graphene nanocomposites by ultrafast pump-probe measurement with pump and probe wavelength in the near-infrared region (800 nm). Our results indicate that the carrier relaxation time of the MoS 2 /graphene nanocomposites is longer than that of graphene due to the existence of a covalent donor-acceptor structure between MoS 2 and graphene. We have further studied the nonlinear absorption through the open aperture Z-scan system from the visible (400 nm) to the near-infrared (1560 nm) broadband frequency range. These findings suggest that MoS 2 /graphene nanocomposites could potentially be used as a passive Q-switcher and mode-locker in ultrafast lasers and ultrafast optical switches.
Inspired by the excellent performance of graphene based saturable absorbers (SAs), many other graphene-like 2D nanomaterials, such as topological insulators (TIs, for example, Bi 2 Se 3   38   , Bi 2 Te 3  39 and  Sb 2 Te 3   40,41 ), and transition metal dichalcogenides (TMDs, for example, MoS 2 24,42 ), have also been widely considered as candidate materials for new types of SAs. The use of these materials has led to the generation of ultrashort pulse with improved performance.
MoS 2 /graphene nanocomposites based optical SAs were fabricated and introduced into an erbium-doped optical fiber laser setup, which could deliver either a passively Q-switched or mode-locked fiber laser operation. The superiority of MoS 2 /graphene nanocomposites will facilitate potential applications of 2D materials for ultrafast photonics, such as Q-switcher, passive laser mode locker, and optical switcher.

Results
Characterizations of MoS 2 /graphene nanocomposites. The characterization of MoS 2 /graphene nanocomposites samples is summarized in Fig. 1. Figure 1(a) shows photographs of the MoS 2 solution (1), graphene (2) and MoS 2 /graphene nanocomposites (3) dispersed uniformly in N-methyl-2-pyrrolidone (NMP), whereas the inset shows the black solid MoS 2 /graphene nanocomposites. The transmission electron microscope (TEM) image of MoS 2 /graphene nanocomposites ( Fig. 1(b)) reveals a general trend with regard to the sheets of MoS 2 homogeneously loaded on graphene. The MoS 2 layers (black stripes) could be visibly identified and appear to be kept in close contact with graphene layers. The selected area electron diffraction (SAED) pattern is shown in the inset of Fig. 1(b). Moreover, distinct electron diffraction patterns from the (100) and (110) planes of graphene and (100), (110) and (118) planes of MoS 2 also confirmed the simultaneous appearance of both graphene and MoS 2 . The UV-Vis spectra were used to demonstrate that the MoS 2 /graphene nanocomposites had been successfully synthesized. As shown in Fig. 1(c), the spectral peaks of the MoS 2 /graphene nanocomposites (red solid line) at 610 and 670 nm correspond to the two characteristic absorption peaks of MoS 2 (blue dotted line), whereas, the increase of light absorption at wavelengths shorter than 500 nm is due to the intrinsic absorption from graphene (black dashed line). Raman spectroscopy was carried out to further probe the internal structures of MoS 2 /graphene nanocomposites. The Raman spectrum ( Fig. 1(d)) shows the existence of typical E 2g 1 , A 1g , 2 LA (M), D and G peaks at 380, 407, 460, 1342 and 1593 cm −1 , respectively. The in-plane E 2g 1 peak results from the opposing vibration of the two S atoms with respect to the Mo atom, whereas the A 1g peak is associated with the out-of-plane vibration of only the S atoms in opposite directions 43 . The asymmetric 2 LA (M) peak is associated with the second-order longitudinal acoustic mode at the M point 44 . These results suggest that the structure is relatively undistorted MoS 2 45 . From the peak distance (27 cm −1 ) between the E 2g 1 and A 1g modes, the structure of the MoS 2 in the nanocomposites has been suggested to consist of multiple layers 46,47 . The D and G peaks confirmed the presence of graphene within the nanocomposites. All of the above characterizations could effectively illustrate the formation of the MoS 2 /graphene nanocomposites.

Ultrafast and nonlinear optical properties. Ultrafast pump-probe and open aperture Z-scan meas-
urements were used to investigate the ultrafast and nonlinear optical properties of the MoS 2 /graphene nanocomposites. The ultrafast carrier dynamics play a critical role in MoS 2 /graphene nanocomposites based optoelectronic applications 48 . The time-dependent transmissivity changes (Δ T/T) of graphene and MoS 2 /graphene nanocomposites at room temperature are shown in Fig. 2. Here, Δ T/T is defined as the relative change of the probe transmissivity caused by the pump: where, T pump and / T w opump are the transmissions of the probe with and without the presence of the pump, respectively. As a reference, the pump-probe measurement was first performed on graphene. As shown in Fig. 2(a), the ultrafast carrier dynamics is consistent with previous reports [9][10][11] . The positive Δ T/T is ascribed to the Pauli blocking induced by excitation based on hot carrier dynamics. Due to the identical wavelengths of the pump and probe pulses, upon the pump excitation, photon-induced carriers or excitons will occupy the probe transition states and reduce the probe photon absorption, leading to a positive Δ T. The time-dependent Δ T/T curve can be successfully fitted to a bi-exponential decay function: where, τ 1 is the decay time with the respective amplitude weights A i 11 . All of the fitting results are summarized in Table 1. According to hot carrier dynamics in graphene, the initial fast relaxation time constant τ 1 (0.47 ± 0.03 ps/0.51 ± 0.03 ps) corresponds to the coupling between the excited charge carriers and optical phonon mode (carrier-phonon scattering) 9 . The slower constant τ 2 (1.7 ± 0.1 ps/2.6 ± 0.1 ps) is attributed to the cooling time of the thermalized carrier-phonon coupling system and electron-hole recombination 11 , which is dependent on the pump intensity. The order of magnitude of the relaxation times obtained in our experiment corresponds well with previously reported observations [9][10][11] .
A similar positive Δ T/T was also observed in the MoS 2 /graphene nanocomposites. However, the carrier relaxation time of the nanocomposites is considerably longer than that of graphene (Fig. 2). The fast and slower relaxation time constants are 1.3 ± 0.2 ps/1.4 ± 0.2 ps τ ( ) 1 and 36 ± 2 ps/37 ± 2 ps τ ( ) 2 , respectively. The carrier dynamics under different pump intensities was investigated to understand the slower relaxation mechanism in the nanocomposites. As shown in Fig. 2(b), there is no relaxation time dependence on the pump power intensity in the nanocomposites. However, the carrier relaxation of graphene is typically dependent on the pump-induced carrier density, higher pump power leads to a slower observed relaxation time 9 . Therefore, carrier-optical phonon interactions in pristine graphene are excluded from the carrier dynamics in the nanocomposites. The inset of Fig. 2(b) shows that the maximum value of the transmissivity change (Δ T/T) is proportional to the pump power intensity when the intensity is below 0.8 GW/cm 2 , which indicated all these experiments were conducted in the linear absorption range. Therefore, the saturation of optical transitions is not likely to be the reason for the intensity-independent slower relaxation time in the MoS 2 /graphene nanocomposites. Thus, the interaction between graphene and MoS 2 is considered to play a significant role in their relaxation dynamics mechanism, as illustrated in Fig. 3.
The utility of graphene as a perfect electron acceptor has been proven in many systems [49][50][51] . In the MoS 2 /graphene nanocomposites with the covalent donor-acceptor structure, the Fermi energy of MoS 2 (E F ) is higher than that of graphene. Therefore, MoS 2 can act as an electron donor through covalently bonding with graphene, thus facilitating the electron transfer from MoS 2 to graphene. The details are described as follows. Upon photoexcitation with the 1.55 eV (800 nm) pump pulse, electrons can be rapidly transited from the valence band to the conduction band, in both graphene with its gapless band structure 7 and multilayer MoS 2 with its indirect gap energy of below 1.48 eV 28 . The excited state electrons of MoS 2 can be efficiently transferred to graphene, due to the slower excited state electron relaxation (hundred ps) of the pristine MoS 2 [52][53][54][55] . This photon-induced electron transfer process may dominate and further disrupt the carrier relaxation occurring at the graphene, which could be the main cause of the slower delay time observed in the MoS 2 /graphene nanocomposites.
The open-aperture Z-scan measurements were also made under variable optical powers from the visible to the near-infrared broadband frequency range. A piece of β -BaB 2 O 4 (BBO) crystal was used to double the frequency of the incident femto-second laser pulse at 800 nm through Second Harmonic Generation (SHG), allowing for the generation of laser pulses at 400 nm. In Fig. 4(b), the beam waist radii of the focused Gaussian beams were approximately 5 and 25 μ m, in the open-aperture Z-scan   Fig. 2.
measurements at 800 and 400 nm, respectively. Figure 5(a,b) show the typical saturable absorption curves of the intensity-dependent open-aperture Z-scan measurements, when the MoS 2 /graphene nanocomposites were moved along the beam focus, indicating the clear dependence of the transmittance peak on the input intensity.
To verify whether MoS 2 /graphene nanocomposites show saturable absorption at near-infrared wavelengths, a home-made Erbium-doped femtosecond fiber laser (central wavelength: 1562.6 nm, repetition rate: 20.13 MHz and pulse duration: 565 fs) was also used as an excitation source. After being focused by a 10 cm focal-length lens, the laser beam was measured to be approximately 10 μ m in diameter at the focal point. The saturable absorption curves were identified at different input intensities, as shown in Fig. 5(c).    The Z-scan measurement results are fitted by the equation 25 : where, T is the transmittance, L is the sample length, α L 0 is the modulation depth, I 0 is the peak intensity, Z 0 is the diffraction length of the beam and I s is the saturable intensity.
From these nonlinear optics fitting curves at different wavelengths, the saturable intensity and modulation depth are summarized in Table 2. Our results indicated that MoS 2 /graphene nanocomposites can be considered to be a broadband saturable absorber with an operation regime from the visible to the near-infrared. Meanwhile, compared with graphene and MoS 2 , the MoS 2 /graphene nanocomposites possess equivalent saturable intensity along with larger modulation depth. The larger modulation depth indicates that the MoS 2 /graphene nanocomposites may be a promising candidate for passive laser Q-switcher and mode-locker for the generation of ultrashort pulse.

Q-switched and mode-locked fiber laser. The broadband saturable absorption of the MoS 2 /
graphene nanocomposites has allowed fabrication of a new type of 2D nanocomposites-based optical fiber saturable absorber device, as shown in Fig. 4(c). Once the pump power exceeded a threshold of 50 mW, stable Q-switching operation self-started immediately without any adjustment of the polarization controller (PC). The typical Q-switching characteristics at a pump power (Pp) of 80 mW are detailed in Fig. 6. Figure 6(a) shows the pulse train with a repetition rate (Rep) of 9.838 kHz, in which the uniform intensity distribution indicates the good stability of this output pulse train. To further investigate the pulse details, we measured the corresponding single pulse profile with a narrower sweep span as shown in Fig. 6(b). The pulse had a full width at half maximum (FWHM) of 11.05 μ s without any random modulations on the top of the pulse, suggesting the fine suppression of Q-switching instability. The corresponding output spectrum is shown in Fig. 6(c). Our Q-switched fiber laser started oscillations at a wavelength of 1567.2 nm. We have also investigated the evolution of Q-switching pulse trains with increasing the pump power, as shown in Fig. 6(d). The results illustrate that the pulse interval gradually deceases with an increase in pump power, which is a typical characteristic of the Q-switching state.  Table 2. NLO parameters of MoS 2 /graphene composites and graphene fitting from the experimental data analysis in Fig. 4: I s : saturable intensity; a 0 L: modulation depth. Moreover, the pulse trains always maintain a uniform intensity distribution. These experimental results demonstrate the successful performance of our fiber laser. Furthermore, we have studied the evolution of the Q-switching parameters with an increase in pump power. The details are given in Fig. 7. The initial Q-switching pulses had a pulse width of 19.12 μ s, a repetition rate of 6.312 kHz and an output power of 464 μ W, corresponding to pulse energy of 73.5 nJ. As the pump power is increased, the pulse duration decreased nonlinearly, finally reaching approximately zero. However, the output average power and repetition rate increased nearly linearly. Q-switching operation could be maintained until the pump power exceeded 165 mW, with a maximum output power of 2.16 mW, a pulse repetition rate of 21.9 kHz and corresponding pulse energy of 98.6 nJ. The minimum pulse duration is approximately 9.31 μ s.
Aside from the passive Q-switching operation, we were also able to demonstrate the passive mode-locking operation with another relatively thin MoS 2 /graphene nanocomposites-based SA. Mode-locking started once the pump power exceeded a threshold of 70 mW. Figure 8 summarizes the characteristics of the mode-locked pulses at a pump power of 80 mW. Figure 8(a) shows its pulse train with a uniform intensity distribution. The repetition rate of 3.47 MHz revealed by this data precisely matches the time interval of 285.2 ns. Figure 8(b) presents the corresponding RF spectrum with a span  of 20 MHz and a resolution bandwidth of 100 Hz. The repetition rate of the soliton pulses is 3.47 MHz, corresponding to the cavity length of ~57.49 m. The signal-to-noise ratio (SNR) of ~53.7 dB is an indication of good mode-locking stability. Figure 8(c) shows that the realized laser output started oscillating at a wavelength of 1571.8 nm with 3-dB bandwidth of 1.5 nm. As a typical characteristic of conventional soliton pulses, the Kelly sidebands have been shown at both sides of the spectrum. A corresponding auto-correlation trace with a full width at half maximum (FWHM) of 3.67 ps is shown in Fig. 8(d). The actual pulse duration is 2.2 ps with an assumed sech 2 pulse profile. The time-bandwidth product (TBP) of the pulses is ~0.405, indicating that the output pulses are slightly chirped. The above-mentioned experimental results also support the excellent performance of our fiber laser. Multiple soliton operations were observed when the pump power was increased to 120 mW.

Discussion
In the pump-probe measurements, the MoS 2 /graphene composites exhibit a fast delay time constant in the range of 1.3-1.4 ps and a slower constant in the 36-37 ps range. According to the slower delay time than that of graphene, we consider that graphene and MoS 2 formed a covalent donor-acceptor structure. Indeed, during the excitation of the MoS 2 /graphene nanocomposites, there is an energy transfer from the excited states of MoS 2 to graphene followed by the non-radiative relaxation of graphene's excited state carriers to the ground state. Due to the excited state electron relaxation time of graphene and MoS 2 , we speculate that the carriers reach the ground state at picosecond relaxation times. Thus, the strong absorption of the nanocomposites and the immediate energy transfer from MoS 2 to graphene could influence the excited state absorption cross section of graphene and further enhance the light-induced bleaching effect. Therefore, their covalent donor-acceptor structure is considered to be important in the mechanisms of the enhancement of nonlinear saturable absorption in the MoS 2 /graphene nanocomposites.
According to the open aperture Z-scan measurements at various wavelengths from the visible to the near-infrared range, the MoS 2 /graphene nanocomposites are experimentally found to possess the enhanced and broadband saturable absorption. Taking advantage of their excellent saturable absorption property, a novel optical saturable absorber device based on MoS 2 /graphene nanocomposites was fabricated. The photonics application for ultra-short pulsed laser operation at the telecommunication band (1.5 μ m) was also demonstrated. Our work provides a new type of tunable 2D photonics materials by combining the optical advantages of different 2D materials. We foresee that their widespread applications for ultrashort pulse generation or optical communications will be increasingly compelling and promising.

Methods
Preparation of MoS 2 /graphene nanocomposites. As a precursor, graphene oxide (GO) was synthesized from graphite powder (> 99.8%, Sinopharm Chemical Reagent Co. Ltd) by the modified Hummer's method by using a mixture of H 2 SO 4 , NaNO 3 , and KMnO 4 56 . The MoS 2 /graphene nanocomposites were obtained via a hydrothermal method 57 . 30 mL of GO brown colloidal dispersion (1 mg/mL) was prepared by ultrasonication for more than 1 h. Subsequently, 20 mL of 70 mmol cationic surfactant (dodecyltrimethylammonium bromide) solution was added to the resulting GO aqueous dispersion and stirred at room temperature for 12 h to form a mixture. The prepared solution was mixed with 20 mL of a mixed solution of 1.5 mmol Na 2 MoO 4 ·2H 2 O and 7.5 mmol L-cysteine (Lcys) with continuous stirring for 30 min. The suspension was then transferred into a 100 mL Teflon-lined stainless steel autoclave and tightly sealed at 240 °C for 24 h. Finally, after being washed several times with deionized water and ethanol, the as-synthesized product was collected by centrifugation, and then naturally cooled and dried in the vacuum oven at 80 °C for 12 h. The obtained black solid product was then annealed at 800 °C for 2 hours with 10% hydrogen in nitrogen flow at 200 sccm to yield the solid MoS 2 /graphene nanocomposites.
In our experimental measurements, MoS 2 is a pristine nanoflake solution purchased from Graphene Supermarket. The graphene from Nanjing SCF Nanotech was dispersed in N-methyl-2-pyrrolidone (NMP) at a concentration of 2 mg/mL and the solid MoS 2 /graphene nanocomposites were dispersed in NMP at the same concentration.

Experimental setups of the pump-probe and open aperture Z-scan measurements. A full
Ti: sapphire amplified laser system (Libra-S, Coherent) with pulse repetition rate of 1 kHz, a center wavelength at 800 nm, and a pulse width of approximately 100 fs was used as the excitation source. The experimental setup of the pump-probe measurement is shown in Fig. 4(a). The output was split into two beams: the intense portion as the pump beam to generate the photo-induced carriers, and the other weak portion as the probe beam to monitor the changes of the transmittivity of the samples at various delays of the probe pulses relative to the pump pulses. The delay time between the two pulses was controlled by a translation stage (Zolix, KSA300-12-X). The pump and probe beams were focused to spot sizes with diameters of approximately 600 and 100 μ m, respectively. The pump and probe beams are orthogonally polarized by placing a half-wave plate in the probe arm and rotating its polarization by 90°. A Glan-Taylor polarizer (G) was placed in front of the detector and set at the same polarization direction as the probe beam to minimize the scattered pump noise. The pump beam was modulated at a frequency Scientific RepoRts | 5:16372 | DOI: 10.1038/srep16372 of 290 Hz by an optical chopper. The transmittance changes of the probe pulses were measured with a photoelectric detector connected to a lock-in amplifier (SR850).
The Z-scan experimental setup is shown in Fig. 4(b). Different types of laser setups were employed as the laser excitation source to fully characterize the broadband nonlinear optical response of the MoS 2 / graphene nanocomposites. The incident laser pulses were also divided into two beams: the reflected beam was used as the reference, and the transmitted beam was focused by a lens as the excitation. The beam waist radius of the focused Gaussian beam was determined by a CCD camera. The pulse powers of the transmitted and reference beams were simultaneously measured by a dual-channel power meter (Newport).
Both pump-probe and open-aperture Z-scan measurements were performed on graphene, MoS 2 and the MoS 2 /graphene nanocomposites dispersion prepared in 1 mm-thick quartz cells. A comparison experiment on the pure solvents (NMP) was carried out, and no clear signal was observed for either pump-probe or Z-scan experiments. Meanwhile, the reproducibility of the transmission curves was tested by cycling the laser intensity up and down to further ensure that the detected materials were free of optical damage.
MoS 2 /graphene nanocomposites-based SA fabrication. The MoS 2 /graphene nanocomposites dispersion in NMP was drop-casted twice directly onto a fiber end-facet, which was then inserted in a standard FC/PC fiber connector for Q-switching operation. To achieve mode-locking operation, we drop-casted the dispersion only once onto another fiber end-facet so that the obtained SA was relatively thin. Allowing the fiber end-facets to dry at room temperature for approximately 10 h allowed us to obtain an even self-assembly of the MoS 2 /graphene nanocomposites molecules onto the fiber end-facets, as shown in Fig. 4(c). The MoS 2 /graphene nanocomposites-based SA device was successfully constructed for the fiber laser application by connecting the MoS 2 /graphene nanocomposites-on-fiber component with another clean, dry FC/PC fiber connector. Figure 4(c) illustrates the schematic setup of the passive Q-switched and mode-locked fiber laser. Through 980/1550 nm wavelength division multiplexer (WDM), the pumping light from a 975 laser diode (LD, 500 mW) was coupled into a piece of 0.9 m highly-doped erbium-doped fiber (EDF, LIEKKI Er 80-8/125) as gain medium. A 10% coupler was used as the output port. Besides the gain fiber, all the remaining fibers used in the cavity were all standard single mode fiber (SMF, SMF-28) with total cavity length of approximately 57.55 m. The Polarization-Independent Isolator (PII-ISO) and Polarization Controller (PC) were used to maintain the unidirectional operation and optimize the performance of our fiber laser, respectively.