The saturable absorber (SA), as a natural light modulation device, is the key part of the pulsed fiber laser in Q-switching and mode-locking. Semiconductor saturable absorber mirrors (SESAMs) are the most well-researched and mature SAs, and we attribute the success in application to the controllable absorption wavelength, saturation threshold, modulation depth, and relaxation time.^[1,2] However, the relatively high cost and narrow working bandwidth of SESAM have urged researchers to seek new alternatives for them. Then, the rise of graphene has meant two-dimensional (2D) materials are gradually being used more and more.^[3] Some of the most representative materials that can be used as SAs have been studied in depth.^[4,5] The unique zero band gap structure of graphene impels it to own the wider working wavelength than other 2D materials. The ultrafast recovery time and strong nonlinearity of graphene further prove that graphene can be a powerful candidate for SA.^[6–9] Topological insulators (TIs) are also interesting because of their large modulation depth and nonlinearity.^[10–13] The superiorities of TIs in light modulation contribute to their great potential in optoelectronics applications. Black phosphorus (BPs) maintain a direct band gap whether they are a bulk or layered structure. Their band gap is adjustable from 0.3 eV to 1.5 eV, which indicates that it is easy to make a breakthrough in the application of photon and photoelectron in the near infrared band.^[14–18] In addition to these classic materials, some novel 2D materials have also begun to arouse interest due to their unique electronic and optical characteristics. BP quantum dots (BPQDs), which are able to combine the quantum confinement and edge effects, exhibit excellent nonlinear optical response according to previous studies.^[19] Moreover, BPQDs have been successfully used in mode-locked erbium-doped fiber laser (EDFL). MXene, as another kind of new member of the 2D material family, shows good conductivity, tunable bandgap, and ability to perform ion intercalation. It has reported that MXene performs optical manipulation in photonic applications, similar to or better than graphene.^[20]

In transition metal dichalcogenides (TMDs), a layer of M (Mo, W, etc.) atoms is sandwiched between two layers of X (S, Se, etc.) atoms, so the chemical formula of TMDs is generally written as MX₂. The neighbor layers of TMDs are bonded with weak van der Waals force, which indicates layered nanosheets are easily separated from the sample.^[21–36] From previous work, the band gap of TMD varies with the number of layers, and so the consequent layer-dependent electronic and optical characteristics are attracting increasing attention.^[37–39] Among TMDs, MoS₂ is prominent considering its performance in photoelectrons. It is reported that the nonlinear optical response of MoS₂ is better than graphene.^[40,41] The few-layer MoS₂ unambiguously exhibits broadband absorption from previous work.^[42] Furthermore, it is found that the starting Q-switching operation threshold of MoS₂ is small under the same experimental conditions compared with those of the materials of the same kind.^[43] The relaxation time of MoS₂ is estimated at ∼30 fs as mentioned in Ref. [44], which indicates that it has potential for fabricating the ultra-fast electronic devices.

There are various methods to be chosen in the fabrication of SA. The two most representative top-down methods are mechanical exfoliation (ME)^[45,46] with the aid of scotch tapes, and liquid-phase exfoliation (LPE)^[47,48] getting dispersant through ultrasonication and centrifugation. These two methods are convenient and low-cost, but the layer number of prepared nanosheets is random. By comparison, the three bottom-up methods, as the common methods, have better performance in the control of the layer number and the quality of the sample, they are the pulsed laser deposition (PLD) method,^[49,50] chemical vapor deposition (CVD) method,^[51,52] and magnetron sputtering deposition (MSD) method,^[53,54] respectively. From previous reports, the layer number of nanosheets prepared by the CVD method tends to be controlled more accurately through adjusting the reaction parameters.

Here in this paper, the MoS₂ produced through the CVD method is moved to the waist of the microfiber to form MoS₂ SA. Finally, the effective area of the taper fiber is fully covered with gold to prevent it from being oxidized. By means of the interaction between the evanescent field of light and the material, the SA is able to avoid some thermal damage.^[55] Furthermore, the long reaction length promotes the MoS₂ SA to fully exhibit the optical nonlinearity. The modulation depth (MD) and saturable intensity of MoS₂ SA are measured to be 11.07% and 1.871 MW/cm², respectively. The proposed MoS₂ based Q-switching EDFL operates at 1530.9 nm. The corresponding pulse duration and maximum output power are 888 ns and 18.8 mW, respectively. The signal-to-noise ratio (SNR) (up to 60 dB) indicates the stability of the laser. The experimental results further prove the favorable optical modulation ability of MoS₂ SA.

The MoS₂ nanosheets were grown on the ultrathin quartz plate by the CVD method. Before the synthesis, the sulfur powder of 0.5 g was placed in the reaction chamber, and the MoO₃ powder of 0.5 g was placed in the heating center, which was at the downstream of sulfur powder. The ultrathin quartz plate was placed at the downstream of the heating center, which was 20 cm away from MoO₃ powder. During the synthesis, the pressure of the chamber was set to be 0.1 Torr (1 Torr = 1.33322 × 10² Pa). Under the drive of Ar flow gas, MoO₃ and sulfur vapors were capable of mixing and reacting fully, as the product was brought to the substrate. Because temperature is a key factor for determining the quality of the finished product, the temperature of the chamber was rising at a constant speed of 25 °C/min. The reaction chamber was heated up to 550 °C and maintained this temperature for 0.5 h, then the reaction was finished.

When the chamber cooled down to room temperature, ultrathin quartz plate attached with MoS₂ film was achieved. To prevent the lattice structure of MoS₂ film from destructing during transfer, the polymethyl methacrylate (PMMA) assisted method was adopted. Before the transfer, the PMMA was spin coated on a large scale over the MoS₂ film. The MoS₂ PMMA was able to be easily separated from ultrathin quartz plate after the open-air drying of the sample. To obtain the pure MoS₂ film, the MoS₂PMMA was etched in acetone. Then the pure MoS₂ film was soaked in deionized water, which could not only remove the remnants, but also make the film float smoothly with the help of water tension. Finally, the MoS₂ film was moved to the conical area of microfiber (SMF-28e), which was observed by using a microscope. The measured diameter of the tapered waist was 12 μm, and the effective contact length between the MoS₂ film and fiber was about 2 mm. After the volatilization of superfluous deionized water, the MoS₂ film closely adhered to the lateral wall of tapered fiber due to van der Waals interaction. Finally, the effective area of the microfiber was fully covered with gold to avoid rapidly oxidizing the material.

With the aid of the atomic force microscope (AFM), an important device for the nano-scale measurement, we obtained the morphology of the MoS₂ surface without damaging the material. Figure 1(a) shows the particles of the material are arranged closely and orderly, which indicates the MoS₂ films prepared by the CVD method are of great uniformity and crystallinity. The comparison of surface height among different regions on the MoS₂ films shows the thickness of MoS₂ film was 30 nm as indicated in Fig. 1(b), which corresponds to 41–42 layers in structure.

Fig. 1.

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	Fig. 1. (color online) (a) Surface morphology image of MoS2 film; (b) altitude difference of the surface.

Raman spectroscopy is considered to be an effective method of determining the specific vibration modes of the material. In Fig. 2, two distinct characteristic peaks are located at 378.9 cm⁻¹ and 403.6 cm⁻¹. According to previous research, the mode at 378.9 cm⁻¹ comes from the inplane vibration which belongs to Mo and S atoms, while the mode at 403.6 cm⁻¹ comes from the out-of-plane vibration which belongs to S atoms.^[56–58] The frequency difference between two peaks illustrates the multilayer structure of prepared MoS₂ film.

Fig. 2.

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	Fig. 2. (color online) Raman spectra of prepared MoS2 films.

The schematic diagram of power-dependent feature measurement is shown in Fig. 3. The light source was a mode-locked fiber laser operating at 1561 nm, of which the pulse duration and the repetition rate were 200 fs and 67 MHz. A variable fiber optic attenuator (VOA) was used to manually adjust the signal attenuation when the optical signal passes through the device. By employing OC, the optical signal could be divided into two parts: one passed through MoS₂ SA to measure its saturated absorptive properties, and the other passed through SMF 28e for comparison. The optical signals at both ends were captured and recorded simultaneously by a power meter. Experimental data could be fitted by where α_s, α_ns, and I_sat are the modulation depth, nonsaturable absorption, and saturation intensity. After the fitting of experimental data in Fig. 4, the saturable intensity and MD of MoS₂ SA are 1.871 MW/cm² and 11.07%, respectively.

Fig. 3.

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	Fig. 3. (color online) Schematic diagram of power-dependent feature measurement.

Fig. 4.

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	Fig. 4. (color online) Intensity-dependent transmission of MoS2 SA, showing its nonlinear absorber characteristics.

In Table 1, the nonlinear parameters of MoS₂ SAs obtained with different fabrication methods are listed to compare the advantages of CVD-MoS₂. By comparison, it can be found that CVD-MoS₂ generally has a large modulation depth, because the materials prepared by the CVD method have better uniformity, which is beneficial to the large modulation depth of SA. In addition, we find that the saturation intensity of CVD-MoS₂ is generally small, which indicates that the laser based on CVD-MoS₂ SA has a lower starting Q-switching operation threshold.

Table 1.

Table 1.

Table 1. Nonlinear parameters of MoS2 SAs obtained with different fabrication methods. . Material Fabrication MD/% Saturation intensity Non-saturable loss/% Ref. MoS2 Scotch-tape micromechanical cleavage technique method 9.70 – 28.80 [59] MoS2 Hydrothermal intercalation/exfoliation approach 10.47 – – [40] MoS2 LPE 4 – 55.90 [60] MoS2 LPE 2 10 MW/cm2 48.56 [61] MoS2 LPE 2.15 129.4 MW/cm2 63.10 [43] MoS2 LPE 6.30 1.6 MW/cm2 18 [62] MoS2 LPE 1.60 13 MW/cm2 54.80 [63] MoS2 PLD 9 27 MW/cm2 40.50 [64] MoS2 CVD 35.40 0.34 MW/cm2 34.10 [65] MoS2 CVD 28.50 0.55 MW/cm2 35 [66] MoS2 CVD 11.07 1.871 MW/cm2 77.61 this work

Table 1. Nonlinear parameters of MoS2 SAs obtained with different fabrication methods. .

An all-fiber ring cavity in Fig. 5 was used in the experiment. Optical devices were coupled into the laser cavity in the order corresponding to the direction of propagation of light. The pump which operated at 976 nm had the maximum allowed output power of 630 mW. The 980/1550 nm WDM successfully coupled the pump light into the ring cavity for exciting the erbium-doped fiber (EDF). The group velocity dispersion (GVD) of EDF (Liekki 110-4/125) was 12 ps²/km at 1550 nm. The intra-cavity polarization and birefringence were mainly optimized by polarization controller (PC). The primary role of an isolator (ISO) is to guarantee unidirectional optical transmission in the cavity to avoid damaging the device, caused by redundant reverse light. The MoS₂ SA was placed between the ISO and WDM. The 20:80 optical coupler (OC) was adopted in the experiment, 20% of which was exported to monitor the real-time experimental data. The monitoring instruments used in the experiment mainly included the RF spectrum analyzer (Agilent E4402B), oscilloscope (Tektronix DPO3054), and optical spectrum analyzer (Yokogawa AQ6370C).

Fig. 5.

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	Fig. 5. (color online) Experimental illustrative diagram of Q-switching EDFL employed with MoS2 SA.

The stable Q-switching pulses occur at 147 mW with the increase of pump power. Meanwhile, the repetition rates of the output pulses with different powers are recorded and exhibited in Fig. 6(a). The tunable range of frequency is 92 kHz–212 kHz. When the pump power is increased to 630 mW, the shortest pulse duration of 888 ns is obtained as shown in Fig. 6(b). The accomplished Q-switching EDFL operates at 1530.9 nm, the corresponding spectral width is 3.14 nm. For a few-hours laser operation, the time-section data of the spectrum is shown in Fig. 6(c). The almost invariable spectral shape indicates the stability of the laser. The RF spectrum of Q-switching EDFL is provided in Fig. 6(d). The SNR of the fundamental frequency increases up to 60 dB, which indicates that the operative condition of the laser is relatively stable. Figure 6(d) shows that the downward trend of frequency multiplication is uniform, which further illustrates the stability of the laser.

Fig. 6.

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	Fig. 6. (color online) Experimental date of Q-switching EDFL employed with MoS2 SA, showing (a) Q-switching pulse trains at different powers, (b) single pulse waveform of Q-switching EDFL, (c) spectra at different times, and (d) RF spectrum of Q-switching EDFL.

According to the experimental monitoring data at different powers, the variation tendency of pulse duration and frequency rate with input power are shown in Fig. 7(a). In the prophase of the power growth, the pulse duration changes more sharply. However, in the later stage of power growth, the pulse duration tends to be stable. This illustrates that MoS₂ SA is close to saturation at high power. With the increase of power, the repetition rate is almost increased uniformly. Similarly, we record and calculate the corresponding output power and pulse energy of the Q-switched pulse train varying with pump power in Fig. 7(b). On the whole, not only output power but also pulse energy increase evenly with input power. The ultimate output power and pulse energy are obtained to be 18.8 mW and 88 nJ, respectively. The maximum damage threshold is about 19.5 mJ/cm².

Fig. 7.

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	Fig. 7. (color online) (a) Variation of pulse duration and frequency rate with pump power, and (b) variation of output power and pulse energy with pump power.

Performance comparisons of Q-switched EDFL based on different SAs are shown in Table 2. By comparison, the CVD-MoS₂ has certain potentials and advantages in realizing short pulse duration. Moreover, the SNR of most Q-switched EDFL is below 50 dB. The SNR of Q-switched EDFL based on CVD-MoS₂ increases up to 60 dB, which indicates the stability of operation. These results show that the microfiber-MoS₂ SA prepared by the CVD method is able to support stable Q-switching operation, which means it can serve as an impactful nonlinear control device for Q-switching EDFL.

Table 2.

Table 2.

Table 2. Performance comparisons of Q-switched EDFL based on different SAs. . Meterials αs/% τ/μs Frequency/kHz Energy/nJ SNR/dB Output power/mW Ref. MoS2 4 3.53 72.74–86.39 74.93 51.6 6.47 [60] WS2 4.48 0.16 91–318 54.4 40 17.3 [67] WSe2 3.02 4.1 46.3–85.4 484.8 41.9 3.16 [43] MoSe2 – 30.4 16.9–32.8 57.9 – 1.9 [68] Bi2Se3 4.3 1.9 495–940 23.8 50 22.35 [69] BP 18.55 10.32 6.983–15.78 94.3 45 1.5 [45] MoS2 11.07 0.888 92–212 88 60 18.8 this work

Table 2. Performance comparisons of Q-switched EDFL based on different SAs. .

By combining the CVD method with the PMMA assisted method, the MoS₂ SA based on microfiber is successfully produced. On the one hand, the films prepared by the CVD method are of good uniformity. On the other hand, the gold-plated SA based on microfiber has some advantages in preventing heat-resistant damage, and anti-oxidation. The saturate intensity and MD of MoS₂ SA are 1.871 MW/cm² and 11.07%, respectively. Moreover, the obtained MoS₂ SA is successfully applied to the Q-switching EDFL. The shortest pulse duration of laser is 888 ns, and the tunable range of frequency is 92 kHz–212 kHz. The ultimate output power and pulse energy are 18.8 mW and 88 nJ, respectively. The SNR of laser increases up to 60 dB, which illustrates the stability of the operation. These results show that the microfiber-MoS₂ SA prepared by the CVD method is able to support stable Q-switching pulse output, and it can serve as an impactful nonlinear control device for Q-switching EDFL.