Dissipative Soliton Generation From Yb-Doped Fiber Laser Modulated by Mechanically Exfoliated NbSe2

We experimentally demonstrated that the stable dissipative solitons can be generated from the Yb-doped fiber laser with the interaction of mechanically exfoliated NbSe2 and the evanescent field of the D-shaped fiber. The modulation depth of the saturable absorber with exfoliated layered NbSe2 is 17.26% and the saturation intensity is 34.8 kW/cm2, respectively. With the proposed saturable absorber, the Yb-doped fiber laser can deliver stable dissipative soliton with pulse duration 174 ps, repetition rate 14.7 MHz and signal-to-noise ratio 59.8 dB at the pump power 300 mW. The experimental results provide an insight for the ultrafast non-linear optical response of layered transition metal dichalcogenide, which may provide strategies for developing high-performance ultrafast non-linear optical devices.


INTRODUCTION
The high-power picosecond Yb-doped fiber lasers have emerged not only as an alternative for industrial and scientific applications, but also as a research platform for soliton dynamics [1][2][3][4]. With the excellent features of high efficiency, broad gain bandwidth, and compact configuration, Yb-doped fiber lasers are currently the laser system of choice for the laser processing of materials, such as welding, drilling and precision cutting [5]. In addition, the energy scaling of fiber laser depends strongly on the non-linearity and cavity dispersion management, and thus the Yb-doped fiber laser usually operating in the normal dispersion regime can provide a platform to investigate the generation of the dissipative soliton and its dynamics [1,3,6]. In view of the compact structure, stable performance and other excellent properties, passively mode-locked Yb-doped fiber lasers modulated by saturable absorber (SA) have aroused extensive attention. With the evolution of the non-linear optical materials, such as graphene [7][8][9], transition metal dichalcogenides (TMDs) [10][11][12][13], topological insulators [14][15][16], black phosphorus [17,18], carbon nanotubes [19][20][21], MXenes [22][23][24], pulsed lasers are being investigated more extensively to meet versatile requirements. However, the preparation and transfer processes inevitably lead to poor repeatability and reliability, and it has been challenging to tune the intrinsic non-linear optical response of the low-dimensional materials [6]. Therefore, finding the suitable materials remain an important step toward developing high-performance ultrafast lasers. Particularly, among the layered materials, the TMDs have been investigated most actively for its excellent non-linear optical performance and versatile material options.
Niobium Diselenide (NbSe 2 ), one kind of TMDs, is known as a prototypical charge-density wave (CDW) material [25][26][27][28][29][30] with superconducting performance [27,[31][32][33][34][35], which involves separate fundamental physical research and applications. NbSe 2 can also be in contact with other semiconductor materials to form lateral or vertical heterojunctions, which can improve the mobility of the device and thus be used to prepare highefficient field effect transistors [36,37]. Moreover, NbSe 2 exhibit a remarkable optical response under different wavelengths and intensity excitations [38,39]. When it comes to the non-linear optics regime with increasing incident intensity, the non-linear optical response of NbSe 2 quantum dots and nanoparticles have been investigated via the mode-locked Ybdoped, Er-doped fiber lasers [40], and Tm-doped fiber laser generation [41]. However, the non-linear optical response and applications of the few-layer NbSe 2 have not been explored, which can broaden the understanding of NbSe 2 in reduced dimensionality and the application in non-linear optics and ultrafast photonics.
Here, we prepared the few-layer NbSe 2 by mechanical exfoliation method, and investigated its non-linear optical absorption characteristics at a wavelength around 1 µm. Due to the interaction between few-layer NbSe 2 and the evanescent field of the D-shaped fiber, the stable modelocked picosecond Yb-doped fiber laser has been delivered successfully with a signal-to-noise ratio of 59.8 dB at wavelength of 1036 nm.

MATERIAL CHARACTERIZATIONS
The few-layer NbSe 2 was prepared by mechanical exfoliation method from its bulk counterpart. The Raman spectroscopy was used to characterize the NbSe 2 sample, as shown in Figure 1A, and the peaks locate at 217.1 and 234.2 cm −1 , the same as previously reported results [42,43]. A spectrophotometer (Shimadzu UV-3600Plus) was used to measure the linear absorption curve of layered NbSe 2 , as shown in Figure 1B. From visible to near-infrared, the few-layer NbSe 2 shows a decreasing absorption, and the absorption becomes weak beyond 1,200 nm. The scanning electron microscope (SEM) picture in Figure 1C shows that the exfoliated NbSe 2 exhibits a layered structure and relatively large area. Figure 1D shows the characterization results of selective area electron diffraction (SAED), which shows that the sample has good crystallinity. The energy dispersive spectrometer (EDS) characterization of the sample is shown in Figure 1E, where only Nb and Se atoms are present, with Cu atoms indicates the metal screen. Furthermore, the atomic ratio of Nb/Se is 0.56 (≈1/2), indicating that the sample is NbSe 2 . Figure 1F shows the atomic force microscope (AFM) characterization of the nanosheets, which shows that the thickness of the nanosheets is about 13 nm. The NbSe 2 is about 9-10 layers considering that the thickness of the monolayer of  NbSe 2 is about 1.1 nm [44], showing the few-layer nature of the NbSe 2 .
The non-linear optical measurements of NbSe 2 have been realized using open aperture Z-scan technique [45]. The wavelength, pulse duration, repetition rate of the light source used in the experiments are 1,064 nm, 4 ns, 100 kHz, respectively. Figure 2A shows the measured Z-scan result of the layered NbSe 2 , which exhibits the saturable absorption behavior with an upward peak at the beam focus. The experimental results have been fitted using a non-linear transmission function, as shown in Figure 2B. The relationship between light transmittance T and incident light intensity I is where α s is the modulation depth, α ns is the nonsaturable components, and I sa is the saturation intensity [46]. Fitting the results, the modulation depth is 17.26% and the saturation intensity is 34.8 kW/cm 2 , respectively. The low saturation intensity resulting from the metallic layered NbSe 2 is favorable for the generation of low-threshold laser pulses [47,48].

EXPERIMENTAL RESULTS AND DISCUSSIONS
A ring cavity has been designed to achieve the dissipative soliton output from the mode-locked Yb-doped fiber laser based on layered NbSe 2 SA, as shown in Figure 3. The length of the entire ring cavity is about 13.6 m long, which contains an ytterbium-doped fiber (LIEKKI Yb1200-4/125) with a length of 0.7 m (group velocity dispersion of 24.22 ps 2 /km) and a 12.9 m HI-1060 single mode fiber (group velocity dispersion of 21.91 ps 2 /km). The pump is a 980 nm laser diode that can deliver pump laser up to 500 mW. Then, a 980/1,060 nm wavelength division multiplexer (WDM) is used to couple the pump into the ring fiber cavity. The isolator (ISO) is used to maintain the unidirectionality of light and the polarization controller (PC) is used to regulate the birefringence of the cavity. A fiber coupler is used to output 1% of the light. Then, the spectrometer (Ando AQ-6317B) and 4 GHz oscilloscope (DS09404A) are used to monitor the output light.
Initially, there was no sample coverage on the D-shaped optical fiber. As the pump power increased, the mode-locking phenomenon could not be observed on the oscilloscope by adjusting the PCs. After the sample was transferred on the Dshaped fiber, we could observe the mode-locking phenomenon by carefully adjusting the PCs. Figure 4A shows a typical oscilloscope pulse sequence at an input power of 300 mW. The spectral curve is shown in Figure 4B, which shows that the center wavelength is 1036 nm and the 3 dB bandwidth is about 3.4 nm. The time bandwidth product (TBP) is 161.6, which indicates that the fiber laser is highly chirped. At an input power of 300 mW, Figure 4C shows a single  pulse with a full width at half maximum (FWHM) of 174 ps by Gaussian fitting. As shown in Figure 4D, the radio frequency (RF) spectrum shows a distinct peak at 14.7 MHz, which matches the repetition rate of the mode-locked fiber laser. The signal-to-noise ratio (SNR) of the RF spectrum is 59.8 dB, indicating that the output mode-locked pulse has excellent stability.
To verify the long-term stability of the output pulse, we record the spectral line of the output pulse every 10 min under the same test conditions, as shown in Figure 5. The plot shows that neither the intensity nor the peak position of the spectrum has shifted or changed after a long-time measurement, and the bandwidth of the spectrum has not changed. Moreover, there are no additional peaks in the spectrum. Similarly, after 3 weeks, we re-examined the nanosheets for stability and found no significant change in spectrum or peak position, which proves that the output pulse is very stable.
To illustrate the advantages of mechanically exfoliated NbSe 2 as SA, we compared the performance of lasers modulated with NbSe 2 quantum dots and NbSe 2 nanoparticles, as shown in Table 1. Moreover, Table 1 also lists the ultrafast modelocked fiber lasers based on low-dimensional material saturable absorbers that have been studied in recent years. From the table, we can see that compared to the fiber lasers based on saturable absorbers of low-dimensional materials, NbSe 2 exhibits the larger modulation depth, which can help the formation of narrower pulses [49,50]. Compared to NbSe 2 quantum dots SA as well as NbSe 2 nanoparticles SA in the Table 1, we can see that mode-locked fiber laser modulated by mechanically exfoliated few-layer NbSe 2 has the highest signal-to-noise ratio, indicating that the modulation of NbSe 2 can help to achieve more stable mode-locking operation. With the high modulation depth and optimized cavity configuration,

CONCLUSIONS
In summary, we have achieved a stable dissipative soliton output from the all-fiberized passively mode-locked Yb-doped fiber laser based on the few-layer NbSe 2 prepared by mechanical exfoliated method. We have verified the non-linear optical performance of the few-layer NbSe 2 via Z-scan technique, and obtained stable dissipative soliton generation with pulse duration 174 ps, repetition rate 14.7 MHz and signal-to-noise ratio 59.8 dB at the pump power 300 mW. The experimental results provide an insight for the ultrafast non-linear optical response of layered transition metal dichalcogenide, which may provide design guidelines for developing high-performance ultrafast non-linear optical devices.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.