Femtosecond Pulsed Fiber Laser by an Optical Device Based on NaOH-LPE Prepared WSe2 Saturable Absorber

We report on all-optical devices prepared from WSe2 combined with drawn tapered fibers as saturable absorbers to achieve ultrashort pulse output. The saturable absorber with a high damage threshold and high saturable absorption characteristics is prepared for application in erbium-doped fiber lasers by the liquid phase exfoliation method for WSe2, and the all-optical device exhibited strong saturable absorption characteristics with a modulation depth of 15% and a saturation intensity of 100.58 W. The net dispersion of the erbium-doped fiber laser cavity is ~−0.1 ps2, and a femtosecond pulse output with a bandwidth of 11.4 nm, a pulse width of 390 fs, and a single-pulse capability of 42 pJ is obtained. Results indicate that the proposed WSe2 saturable absorbers are efficient, photonic devices to realize stable fiber lasers. The results demonstrate that the WSe2 saturable absorber is an effective photonic device for realizing stable fiber lasers, which have a certain significance for the development of potential photonic devices.


Introduction
The key technology to generate mode-locked ultrashort pulses in ultrafast fiber lasers is the placement of saturable absorption devices in the laser cavity [1,2]. In 2009, Bao et al. proposed that graphene two-dimensional (2D) materials could be used for ultrafast laser mode-locked devices and obtained high-quality graphene mode-locked pulses [3]. Subsequently, research on mode-locked ultrafast fiber lasers with 2D materials has mushroomed. The emergence of new saturable absorbers (SAs) for 2D materials has been shown to create a wave of research in the field of laser technology. The research results have demonstrated that 2D SAs have some unique advantages over conventional commercial semiconductor SAs [3,4], such as broadband saturable absorption, high damage threshold, high modulation depth, etc. [5][6][7]. Currently, 2D SAs, (e.g., topological insulators [8,9], transition metal-sulfur compounds (TMDs) [10,11], MXene [12,13], etc.), have been widely used in fiber lasers to generate ultrashort pulses. Therefore, the multiple functions of 2D materials have driven laser physicists to try to exploit the unique nonlinear optical properties of 2D materials to study the ultrashort pulse generation and soliton dynamics of fiber lasers [14][15][16][17]. Many new 2D nanomaterials are becoming more and more abundant in the optoelectronic research [7,[18][19][20][21][22].
To achieve compatibility of 2D nanomaterial SAs with existing fiber laser systems, several structures have been developed for the integration of SAs, such as fiber end face; tapered fiber; D-type fiber; photonic crystal fiber; free-space coupling substrate, etc. The ideal SA material usually has good stability, ultra-fast relaxation time, and a wide wavelength operating range. SA that can maintain long-term stability in the environment is necessary for mode-locked lasers to be able to operate stably. SA of 2D nanomaterials combination of the LPE method and the light pulse deposition method has been considered an effective strategy to prepare the 2D materials-based SAs [44].
Here, we demonstrate an erbium-doped fiber pulsed laser based on the WSe 2 SAs fabricated by NaOH-assisted LPE and optical pulse deposition. The preparation of WSe 2 is first described and material criteria are performed, followed by the preparation of WSe 2 -SA and verification of the saturable absorption properties of the device. The all-optical device, WSe 2 -SA, has a modulation depth of 15% and a saturable intensity of 100.58 W. The erbiumdoped fiber laser achieves an ultrashort pulse output with a spectral width of 11.4 nm and a pulse width of 390 fs. Our work highlights the potential of all-optical devices prepared from WSe 2 for applications in the field of photonics.

Material
As shown in Figure 1a, the NaOH-LEP method has been employed to prepare the few-layer WSe 2 nanosheets. In brief, the bulk phase of WSe 2 material is placed in a mortar, and the 4 mL NMP solution is added for grinding for 30 min to obtain WSe 2 powder. Then, the 40 mg WSe 2 power is added to 20 mL saturated NaOH solution in NMP. After that, the above dispersion liquid of WSe 2 powder is placed in a sonicator and treated with ultrasonic for 10 h to exfoliate the materials. Notably, the intralayer of the WSe 2 molecule consists of strong covalent bonds, while the interlayer consists of weak van der Waals forces [45]. This means that the interlayer van der Waals forces of WSe 2 can be destroyed by the strong ultrasonic process, thereby realizing the purpose of exfoliating WSe 2 into two-dimensional nanosheets. After sonication proceeding, the WSe 2 dispersion was centrifuged at 3000 rpm for 20 min to remove the unexfoliated WSe 2 . Then, the sediments with unexfoliated WSe 2 are removed, and the supernatant contained with few-layer nanosheets is retained. After that, the supernatant is centrifuged at 18,000 rpm for 30 min. The supernatant was removed by pipette and the precipitate was retained. Finally, the retained precipitate is redispersed in ethanol to obtain the dispersion with few-layered WSe 2 nanosheets. Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 10 charge, and thus causes excellent stability in water. In addition, ions in NMP solution can intercalate into the interlayers of WSe2, thereby enhancing the yield of nanosheets. Furthermore, the combination of the LPE method and the light pulse deposition method has been considered an effective strategy to prepare the 2D materials-based SAs [44].
Here, we demonstrate an erbium-doped fiber pulsed laser based on the WSe2 SAs fabricated by NaOH-assisted LPE and optical pulse deposition. The preparation of WSe2 is first described and material criteria are performed, followed by the preparation of WSe2-SA and verification of the saturable absorption properties of the device. The all-optical device, WSe2-SA, has a modulation depth of 15% and a saturable intensity of 100.58 W. The erbium-doped fiber laser achieves an ultrashort pulse output with a spectral width of 11.4 nm and a pulse width of 390 fs. Our work highlights the potential of all-optical devices prepared from WSe2 for applications in the field of photonics.

Material
As shown in Figure 1a, the NaOH-LEP method has been employed to prepare the few-layer WSe2 nanosheets. In brief, the bulk phase of WSe2 material is placed in a mortar, and the 4 mL NMP solution is added for grinding for 30 min to obtain WSe2 powder. Then, the 40 mg WSe2 power is added to 20 mL saturated NaOH solution in NMP. After that, the above dispersion liquid of WSe2 powder is placed in a sonicator and treated with ultrasonic for 10 h to exfoliate the materials. Notably, the intralayer of the WSe2 molecule consists of strong covalent bonds, while the interlayer consists of weak van der Waals forces [45]. This means that the interlayer van der Waals forces of WSe2 can be destroyed by the strong ultrasonic process, thereby realizing the purpose of exfoliating WSe2 into two-dimensional nanosheets. After sonication proceeding, the WSe2 dispersion was centrifuged at 3000 rpm for 20 min to remove the unexfoliated WSe2. Then, the sediments with unexfoliated WSe2 are removed, and the supernatant contained with few-layer nanosheets is retained. After that, the supernatant is centrifuged at 18,000 rpm for 30 min. The supernatant was removed by pipette and the precipitate was retained. Finally, the retained precipitate is redispersed in ethanol to obtain the dispersion with few-layered WSe2 nanosheets. To confirm the successful preparation of few-layer WSe 2 nanosheets, some characterization methods have been employed to investigate the morphology of as-prepared WSe 2 nanosheets, the results are performed in Figure 1. As shown in Figure 1b, transmission electron microscopy (TEM) images show a few very thin and flexible nanosheets with different thicknesses stacked on the other one, and the as-prepared few-layer WSe 2 nanosheets (lateral dimension is about 600 nm) could be clearly identified with a layered structure. In order to study the thickness and lateral dimensions of the WSe 2 nanosheets more accurately, the atomic force microscopy (AFM) images of the nanosheets were measured. As shown in Figure 1d, the AFM image shows that size of this WSe 2 nanosheet is about 1.2 µm. Height profiling of the nanosheet was shown in Figure 1e, which suggests that the thickness of this WSe 2 nanosheet in Figure 1d marked with line A is 0.8 nm. It is demonstrating that the very thin WSe 2 nanosheets have been prepared by the NaOH-LEP method. The Raman spectrum is carried out to further investigated the molecular structure of WSe 2 nanosheets. Additionally, the Raman spectrum is shown in Figure 1c, two characteristic peaks at 248 cm −1 and 251.5 cm −1 positions can be observed, which corresponds to E 1 2g (in-plane) and A 1g (out-of-plane) modes of WSe 2 . In addition, Raman peaks were observed at 136, 365.7, and 387.6 cm −1 . It is consistent with the Raman spectra reported in previous work [46][47][48][49]. The above results show that few-layer WSe 2 nanosheets have been successfully prepared. In addition to this, the absorption spectrum of WSe 2 is shown in Figure 2, which shows that WSe 2 has a broadband absorption property, which is beneficial for the application of this material in ultrafast lasers. To confirm the successful preparation of few-layer WSe2 nanosheets, some characterization methods have been employed to investigate the morphology of as-prepared WSe2 nanosheets, the results are performed in Figure 1. As shown in Figure 1b, transmission electron microscopy (TEM) images show a few very thin and flexible nanosheets with different thicknesses stacked on the other one, and the as-prepared few-layer WSe2 nanosheets (lateral dimension is about 600 nm) could be clearly identified with a layered structure. In order to study the thickness and lateral dimensions of the WSe2 nanosheets more accurately, the atomic force microscopy (AFM) images of the nanosheets were measured. As shown in Figure 1d, the AFM image shows that size of this WSe2 nanosheet is about 1.2 μm. Height profiling of the nanosheet was shown in Figure 1e, which suggests that the thickness of this WSe2 nanosheet in Figure 1d marked with line A is 0.8 nm. It is demonstrating that the very thin WSe2 nanosheets have been prepared by the NaOH-LEP method. The Raman spectrum is carried out to further investigated the molecular structure of WSe2 nanosheets. Additionally, the Raman spectrum is shown in Figure 1c, two characteristic peaks at 248 cm −1 and 251.5 cm −1 positions can be observed, which corresponds to (in-plane) and (out-of-plane) modes of WSe2. In addition, Raman peaks were observed at 136, 365.7, and 387.6 cm −1 . It is consistent with the Raman spectra reported in previous work [46][47][48][49]. The above results show that few-layer WSe2 nanosheets have been successfully prepared. In addition to this, the absorption spectrum of WSe2 is shown in Figure 2, which shows that WSe2 has a broadband absorption property, which is beneficial for the application of this material in ultrafast lasers.

Devices
We chose the tapered fiber with simple preparation, large specific surface area and strong evanescent field as the substrate for SA. YOFC SMF 28 fiber is selected, and the tapered fiber is prepared by the hydroxide flame method. Potassium hydroxide (KOH) aqueous solution is used as the electrolyte to electrolyze hydrogen and oxygen in the hydrogen-oxygen machine using the principle of electrolysis of water. Hydrogen is used as an oxygen fuel for fuel and ignition produces a hydrogen-oxygen flame. The outer cladding of the fiber is peeled off, and the outer flame of the hydrogen-oxygen flame is used to cauterize this part of the fiber, and the fiber is pulled at high temperature. The size of the flame is controlled during the preparation process and the fiber is drawn at a uniform speed. A tapered fiber with a diameter of 6 μm, a tapered length of ~600 μm, and a loss of 0.1 dB at 980 nm is obtained.

Devices
We chose the tapered fiber with simple preparation, large specific surface area and strong evanescent field as the substrate for SA. YOFC SMF 28 fiber is selected, and the tapered fiber is prepared by the hydroxide flame method. Potassium hydroxide (KOH) aqueous solution is used as the electrolyte to electrolyze hydrogen and oxygen in the hydrogen-oxygen machine using the principle of electrolysis of water. Hydrogen is used as an oxygen fuel for fuel and ignition produces a hydrogen-oxygen flame. The outer cladding of the fiber is peeled off, and the outer flame of the hydrogen-oxygen flame is used to cauterize this part of the fiber, and the fiber is pulled at high temperature. The size of the flame is controlled during the preparation process and the fiber is drawn at a uniform speed. A tapered fiber with a diameter of 6 µm, a tapered length of~600 µm, and a loss of 0.1 dB at 980 nm is obtained.
WSe 2 for this experiment is prepared by the NaOH-LPE method, which prepared the material with certain homogeneity and facilitated the coupling between WSe 2 and the near field of the tapered fiber surface. The WSe 2 -SA is prepared by the optical deposition Nanomaterials 2022, 12, 2747 5 of 10 method, and the depositional procedure of the material is observed in real time under the microscope during the preparation project [50,51]. The light source of deposition is a 980 nm pump light source with 50 mW, and a drop of material is first applied to the tapered area of the tapered fiber. With time, a second drop is applied with time, while the loss is observed, and the drop of material is stopped when the loss is 3 dB. The final measured loss of WSe 2 -SA (shown in Figure 3a) is 3.2 dB at 1550 nm and 3 dB at 980 nm. The maximum power (available) that the device can withstand is 2 W of 980 nm pumped light. method, and the depositional procedure of the material is observed in real time under the microscope during the preparation project [50,51]. The light source of deposition is a 980 nm pump light source with 50 mW, and a drop of material is first applied to the tapered area of the tapered fiber. With time, a second drop is applied with time, while the loss is observed, and the drop of material is stopped when the loss is 3 dB. The final measured loss of WSe2-SA (shown in Figure 3a) is 3.2 dB at 1550 nm and 3 dB at 980 nm. The maximum power (available) that the device can withstand is 2 W of 980 nm pumped light.
The prepared WSe2-SA devices are used in the field of lasers with saturable absorption properties in the third-order nonlinear effect, which is an important property to measure as SA. We measured the saturable absorption properties of the devices using a double detection method [51]. The Z-scan can only illustrate the saturable absorption properties of nonlinear optical materials. For the measurement of the saturation absorption properties of SA, the double detection method is simple, economical, and provides a direct indication of the saturation absorption properties of the entire device. As shown in Figure 3b, the following formula is used to fit the experimental data: is the modulation depth, I is the incident peak power, sat I is the saturation power, and ns T is the non-saturated loss. The modulation depth, saturable power, and non-saturated loss of WSe2-SA are estimated to be 15%, 100.58 W, and 41.5%. It indicates that WSe2-SA has a strong saturable absorption performance at 1550 nm, indicating that the device can be used as an ultrafast optical switch for generating ultrashort pulses at 1550 nm.

Experimental Setup and Results
The erbium-doped fiber ring laser built in this experiment is shown in Figure 4. YOFC 1013 erbium-doped fiber with group velocity dispersion (GVD) of ~9 ps 2 /km at 1550 nm is selected as the gain medium, and a 980 nm/1550 nm wavelength division multiplexer (WDM) with a pigtail of HI1060 and its length of 1 m, corresponding to a GVD of −7 ps 2 /km. The isolator (ISO) within the cavity is polarization independent to ensure unidirectional propagation. A three-paste polarization controller (PC) regulates the intracavity polarization. The 30% output of the optical coupler (OC) is used to measure the performance of the laser. The performance of the fiber laser is measured with the aid of an 18 GHz high-speed photodetector using a spectrum analyzer (OSA, YOKOGAWA AQ6370C, Janpan), oscilloscope (Agilent MSO7054A, America), RF analyzer (Agilent N9320B, America), and commercial autocorrelator (Femtochrome FR-103, America). The prepared WSe 2 -SA devices are used in the field of lasers with saturable absorption properties in the third-order nonlinear effect, which is an important property to measure as SA. We measured the saturable absorption properties of the devices using a double detection method [51]. The Z-scan can only illustrate the saturable absorption properties of nonlinear optical materials. For the measurement of the saturation absorption properties of SA, the double detection method is simple, economical, and provides a direct indication of the saturation absorption properties of the entire device.
As shown in Figure 3b, the following formula is used to fit the experimental data: Equation T(I) = 1 − ∆T × exp(−I/I sat ) − T ns , where T(I) is the transmittance, ∆T is the modulation depth, I is the incident peak power, I sat is the saturation power, and T ns is the non-saturated loss. The modulation depth, saturable power, and non-saturated loss of WSe 2 -SA are estimated to be 15%, 100.58 W, and 41.5%. It indicates that WSe 2 -SA has a strong saturable absorption performance at 1550 nm, indicating that the device can be used as an ultrafast optical switch for generating ultrashort pulses at 1550 nm.

Experimental Setup and Results
The erbium-doped fiber ring laser built in this experiment is shown in Figure 4. YOFC 1013 erbium-doped fiber with group velocity dispersion (GVD) of~9 ps 2 /km at 1550 nm is selected as the gain medium, and a 980 nm/1550 nm wavelength division multiplexer (WDM) with a pigtail of HI1060 and its length of 1 m, corresponding to a GVD of −7 ps 2 /km. The isolator (ISO) within the cavity is polarization independent to ensure unidirectional propagation. A three-paste polarization controller (PC) regulates the intracavity polarization. The 30% output of the optical coupler (OC) is used to measure the performance of the laser. The performance of the fiber laser is measured with the aid of an 18 GHz highspeed photodetector using a spectrum analyzer (OSA, YOKOGAWA AQ6370C, Janpan), oscilloscope (Agilent MSO7054A, America), RF analyzer (Agilent N9320B, America), and commercial autocorrelator (Femtochrome FR-103, America).  When the 980 nm pump power is set to 285 mW, a stable clamping state can be observed by adjusting the PC. Figure 5a shows a pulse sequence with a time interval of 40.8 ns. The average output power of the laser is 1.03 mW, corresponding to a single pulse energy of 42 pJ. The theoretical calculation corresponds to a cavity length of 8.16 m. The actual cavity length is about 8.3 m, which is consistent with the theoretical value. The GDV of the single-mode fiber in the cavity is −23 ps 2 /km, and the net dispersion in the cavity is calculated to be −0.1 ps 2 . Figure 5b shows the output spectrum with a central wavelength of 1552.68 nm and a 3 dB bandwidth of 11.4 nm. Figure 5c shows the pulse width, and after sech 2 fitting, the calculated output pulse width is 390 fs, where the calculated timebandwidth product (TBP) is 0.55. Figure 5d shows a spectral signal-to-noise ratio of ~ 65 dB at a fundamental frequency of 24.5 MHz, with no harmonics. This indicates that the mode-locked fiber laser has good mode-locking performance. These experimental results confirm that WSe2 can be used as a saturable absorbing material to generate stable modelocked pulse output. In the future, the performance of the mode-locked pulses and the properties of WSe2-SA can be customized by continuously optimizing the preparation process of WSe2 and improving the design of the laser cavity.  When the 980 nm pump power is set to 285 mW, a stable clamping state can be observed by adjusting the PC. Figure 5a shows a pulse sequence with a time interval of 40.8 ns. The average output power of the laser is 1.03 mW, corresponding to a single pulse energy of 42 pJ. The theoretical calculation corresponds to a cavity length of 8.16 m. The actual cavity length is about 8.3 m, which is consistent with the theoretical value. The GDV of the single-mode fiber in the cavity is −23 ps 2 /km, and the net dispersion in the cavity is calculated to be −0.1 ps 2 . Figure 5b shows the output spectrum with a central wavelength of 1552.68 nm and a 3 dB bandwidth of 11.4 nm. Figure 5c shows the pulse width, and after sech 2 fitting, the calculated output pulse width is 390 fs, where the calculated time-bandwidth product (TBP) is 0.55. Figure 5d shows a spectral signal-to-noise ratio of~65 dB at a fundamental frequency of 24.5 MHz, with no harmonics. This indicates that the mode-locked fiber laser has good mode-locking performance. These experimental results confirm that WSe 2 can be used as a saturable absorbing material to generate stable mode-locked pulse output. In the future, the performance of the mode-locked pulses and the properties of WSe 2 -SA can be customized by continuously optimizing the preparation process of WSe 2 and improving the design of the laser cavity. When the 980 nm pump power is set to 285 mW, a stable clamping state can be observed by adjusting the PC. Figure 5a shows a pulse sequence with a time interval of 40.8 ns. The average output power of the laser is 1.03 mW, corresponding to a single pulse energy of 42 pJ. The theoretical calculation corresponds to a cavity length of 8.16 m. The actual cavity length is about 8.3 m, which is consistent with the theoretical value. The GDV of the single-mode fiber in the cavity is −23 ps 2 /km, and the net dispersion in the cavity is calculated to be −0.1 ps 2 . Figure 5b shows the output spectrum with a central wavelength of 1552.68 nm and a 3 dB bandwidth of 11.4 nm. Figure 5c shows the pulse width, and after sech 2 fitting, the calculated output pulse width is 390 fs, where the calculated timebandwidth product (TBP) is 0.55. Figure 5d shows a spectral signal-to-noise ratio of ~ 65 dB at a fundamental frequency of 24.5 MHz, with no harmonics. This indicates that the mode-locked fiber laser has good mode-locking performance. These experimental results confirm that WSe2 can be used as a saturable absorbing material to generate stable modelocked pulse output. In the future, the performance of the mode-locked pulses and the properties of WSe2-SA can be customized by continuously optimizing the preparation process of WSe2 and improving the design of the laser cavity.

Discussion
We developed a functional WSe 2 device, verified the saturable absorption characteristics of the device, and finally used it in an ultrafast fiber laser to realize the output of femtosecond pulses. The earliest graphene-based SA has some excellent nonlinear optical properties, but the zero-bandgap structure and low absorption coefficient of graphene weaken its optical modulation ability, thus limiting the light-matter interaction. Similarly, it is shown that MoS 2 has layer-dependent nonlinear optical properties, and MoS 2 has stronger saturable absorption than graphene in the case of a single or few layers. WSe 2 is an important member of TMDs materials. Like MoS 2 , it has the advanced properties of strong light-matter interaction and thickness-tunable bandgap. Compared with MoS 2 , WSe 2 can be more suitably applied to broadband nonlinear optics due to its smaller band gap than MoS 2 . Furthermore, WSe 2 can generate strongly bound and tunable excitons due to its optically initialized valley polarization and coherence. Moreover, multiple studies have shown that WSe 2 has an exceptionally strong exciton binding energy. In addition to this, WSe 2 also has p-type electron transport properties that are completely different from MoS 2 .WSe 2 can be combined with other materials to construct heterojunctions (pn junction) to improve the performance of WSe 2 optical devices, which is attributed to the inherent p-type semiconductor properties and excellent compatibility of WSe 2 . All the above advantages indicate that WSe 2 has excellent nonlinear optical properties and has broad application prospects in fiber lasers.
So far, the WSe 2 -based implementation of erbium-doped fiber lasers with Q-switched pulses is more common, and mode-locked pulses are less common. According to Table 1, the main preparation methods of WSe 2 include CVD and LPE, integrated methods include microfiber, tapered fiber, side-polished fiber, and fiber end face (sandwich structure). The few-layer WSe 2 nanosheets have been prepared by NaOH-LPE method. The larger area and ultrathin WSe 2 nanosheets with high yield can be achieved by our NaOH-LPE method. Our tapered fiber is more convenient and economical to implement, and most importantly, the damage threshold of the device is up to 1.5W, which is not available in the sandwich structure. It can be seen from the Table 1 that the WSe 2 thin films prepared by CVD seem to have better properties. This is attributed to the high uniformity of WSe 2 films prepared by CVD, which is beneficial to obtain better fiber lasers. However, the problems of complex operation, slow growth rate of thin films, unfriendly environments, poor repeatability and high cost limits the application of the CVD method to a certain extent, especially for the preparation of large-scale 2D materials. The simple operation and low cost of LEP make it an effective means for the rapid and large-scale preparation of 2D materials. Moreover, in order to solve the general problem of low yield in the LPE method, saturated NaOH solution was introduced into the system to develop the NaOH-LPE method. In this work, large-scale of WSe 2 nanosheets with big lateral size and very thin thickness were prepared by the NaOH-LPE method and fabricated as SA. For the SA prepared by the NaOH-LPE method, the parameters obtained in this paper are more optimal [36,52,53]. With the advancement of science and technology, the requirements for stability, repeatability, and easy maintenance of fiber lasers are also increasing. Therefore, one of the important parts of the research from the preparation and structural design of saturable absorbers. Combining WSe 2 and plasmonic structures to enhance the light-matter interaction is one of the potential strategies to enhance the performance of WSe 2 -SA. Furthermore, designing and constructing SA based on WSe 2 heterojunctions is one of the other effective strategies. Therefore, we will further carry out the work in the field of WSe 2 -based nonlinear optics around the above two strategies.

Conclusions
In summary, we achieved ultrashort pulses generated by an erbium-doped fiber mode-locked laser based on WSe 2 -SA. The substrate of the all-optical device WSe 2 -SA is a tapered fiber, and the device has a high damage threshold and a strong saturable absorption property. Based on this all-optical device, we achieved stable mode-locked operation at 1552.68 nm with a pulse of 390 fs and a spectral width of 11.4 nm. The net cavity dispersion of the laser is~−0.1 ps 2 . This work highlights the potential of WSe 2 as a member of two-dimensional transition metal materials for applications in photonics.