Passively Q-Switched Erbium-Doped Fiber Laser Based on GeSe Saturable Absorber

GeSe nanosheets were prepared by ultrasonic-assisted liquid phase exfoliation (LPE), and the nonlinear saturated absorption performance was experimen-tally studied. The modulation depth and saturation intensity of the prepared GeSe saturable absorber (SA) were 15% and 1.44 MW/cm 2 , respectively. Using the saturated absorption characteristics of GeSe SA, a passively Q-switched erbium-doped fiber laser was systematically demonstrated. As the pump power increases, the pulse repetition frequency increases from 22.8 kHz to 77.59 kHz. The shortest pulse duration is 1.51 μs, and the corresponding pulse energy is 46.14 nJ. Experimental results show that GeSe nanosheets can be used as high-efficiency SA in fiber lasers. Our results will provide a useful reference for demonstrating pulsed fiber lasers based on GeSe equipment.


Introduction
Compared with continuous light (CW) lasers, ultrafast fiber lasers have shorter pulse duration and higher pulse energy, and have been extensively studied. So far, the use of saturable absorbers (SAs) for passive Q-switching and mode-locking is the main technique to modulate CW into a pulse state [1]- [10]. In the process of exploring new SAs, researchers are paying more and more attention to ideal SA materials with high nonlinearity, fast response time, wide absorption band, ambient atmospheric stability, low cost, and low loss [11]- [32]. In recent years, new materials such as quantum dots [33] [34], metal nanoparticles [35] [36] [37] [ 38], and two-dimensional (2D) materials have gradually entered our field of vision, and have been proven to have excellent saturable absorption characteris-How to cite this paper: Liu, Y., Hong, Z.F., Liu, X.J., Zhang, R. and Yin, S.L. tics. Among them, 2D materials have many unique optoelectronic properties that traditional materials do not have, especially their excellent nonlinear optical properties, which show great potential in manufacturing high-performance and new functional optoelectronic devices. In 2004, people successfully produced a 2D material graphene [39], and proved that it has saturable absorption characteristics. Since then, 2D materials such as topological insulators (TI) [40] [41], black phosphorus (BP) [42] [43], MXenes [44] [45], and transition metal dichalcogenides (TMDs) have attracted widespread attention, and most of them have been used as SA in optical operations.
TMDs, as a new type of functional materials for optoelectronic applications, has recently attracted widespread research interest [6]- [12]. TMDs are layered materials with the general formula MX 2 , where M is a transition metal (Nb, Ta, Ti, Mo, W, ...) and X is a chalcogen element (S, Se, Te). The layers, made of triangular lattices of transition metal atoms sandwiched by covalently bonded chalcogens, are held together by weak van der Waals forces, which is the reason why TMDs can be readily exfoliated into thin flakes down to the single-layer limit. In addition, when the material changes from a multilayer to a single layer, the TMDs will transition from an indirect band gap to a direct band gap, which indicates that the band gap of TMDs can be designed. This feature brings some excellent optical properties, such as high carrier mobility, controllable photoelectron properties and outstanding nonlinear optical absorption [6] [7] [8]. However, current TMDs materials usually show a large band gap in visible light or near-infrared light, so the absorption at mid-infrared wavelengths is relatively weak. In recent years, the layered TMDs nanostructures composed of IV-VI elements such as GeS, GeSe, GeSe 2 , and SnSe have attracted great interest. Their band gap is usually in the range of 0.5 -1.5 eV. GeSe, GeS and other germanium-based layered nanostructures are alternatives to traditional TMDs such as MoS 2 and WS 2 . They are expected to become SAs for fiber lasers due to their relatively high stability and environmental sustainability. Among the materials with the M-X structure, GeSe is the only material with a direct band gap, and the direct and indirect band gaps are closely placed. It belongs to a narrow band gap p-type semiconductor, and its band gap is in the range of 1.1 -1.2 eV [46]. These characteristics make it possible to use SAs as fiber lasers. GeSe crystals are composed of vertically stacked layers, which are held together by weaker van der Waals force. Atoms are connected by covalent bonds with a strong force, which can eliminate dangling bonds and surface states. In addition, GeSe has a highly anisotropic crystal with a layered structure, where the layers are parallel to the growth direction. Because the surface of the GeSe nanosheet is chemically inert, it guarantees its high chemical and environmental stability. In 2013, B. Mukherjee et al. proved that photodetectors based on GeSe nanosheets have parameters comparable to other two-dimensional materials [47].
In this contribution, we used ultrasonic-assisted liquid phase exfoliation (LPE) to prepare GeSe nanosheets. The modulation depth and saturation intensity of GeSe SA prepared were 15% and 1.44 MW/cm 2 , respectively. In addition, we

Preparation and Characterization of GeSe SA
In this work, an ultrasonic-assisted LPE method was used as a simple and low-cost method to prepare GeSe nanosheets. Mix 60 mg of GeSe powder with 60 ml of pure water, and ultrasonically treat at a low temperature for 10 hours to obtain GeSe dispersion. The dispersion was then centrifuged in a high-speed centrifugal device at 3000 rpm for 20 minutes until the GeSe powder was fully dispersed.
Finally, we select the upper 80% solution, as shown in Figure 1, for subsequent experiments.
First, the GeSe-PVA film is manufactured. The preparation process is shown in Figure 2, 400 mg of polyvinyl acetate (PVA) powder was added to 10 ml of GeSe solution, stirred for one hour to fully dissolve the PVA, and the mixture was sonicated for 1 hour to prepare a uniform GeSe-PVA dispersion. The experiment uses condensed PVA, which is a white powdery resin that can be dissolved in water at room temperature and can support GeSe materials well. At the same time, PVA has a good model, the film made is not easy to tear and wear-resistant, and has good permeability in the visible light to infrared light wave range. Therefore, it is easy to form a thin film on the end face of the optical  is shown in Figure 3(b). It's seen that there is no obvious defect, and the lattice distance is about 0.36 nm corresponding to the (2, 0, 1) lattice plane.
We measured the Raman spectrum of GeSe nanosheets at an excitation wavelength of 514 nm, as shown in Figure 4. It can be seen from the figure that GeSe nanosheets have two characteristic peaks, namely the B 2u peak and the A g peak, and their Raman frequency shifts correspond to 149.8 cm −1 and 188.7 cm −1 , respectively. This result corresponds to the reported Raman peak of GeSe, which can well prove that the prepared material is GeSe [48] [49].
In addition, in order to confirm the thickness of the prepared GeSe-PVA film, we measured it with an atomic force microscope (AFM).    where T(I) is transmission, ΔT is the modulation depth, I is input laser intensity, I sat and T ns are saturation intensity and nonsaturable absorbance, respectively.
According to the fitting analysis, the modulation depth and saturation intensity of the prepared GeSe-PVA film are provided to be 15% and 1.44 MW/cm 2 , respectively.
Using UV/VIS/NIR spectrophotometer (U-3500, Hitachi, Japan) was meas- A good SA should have a high modulation depth (~10% for fiber lasers) and a low saturation intensity value. As mentioned above, GeSe SA has a relatively large modulation depth of 15% and a lower saturation intensity of 1.44 MW/cm 2 , which is more suitable for achieving nonlinear saturation absorption of ultrafast pulses.  During the experiment, we increased the pump power while using a spectrometer and an oscilloscope to observe the output characteristics of the pulse in the laser. When the pump power was increased to 25 mW, we observed the continuous wave output in the laser in the oscilloscope. Then, when the pump power reached 47 mW, Q-switched pulse output began to appear in the laser. Throughout the experiment, as the pump power changes, the intensity of each pulse in the corresponding pulse sequence has been kept within a similar range, indicating that the output pulse has good stability. When the pump power is increased to the maximum value of 515 mW, the Q-switched pulse output is still stable. Figure 9 shows the pulse output characteristics when the pump power is 515 mW. Figure 9(a) is a single pulse sequence diagram. From the figure, we can see that there are some spikes at the top of the pulse, which causes the pulse envelope to be not smooth enough, but no jitter is found in the oscilloscope. The shortest pulse duration when the pump power is 515 mW is 1.51 μs. The illustration shows the output pulse sequence at this time, the intensity of each pulse is basically the same, and the repetition frequency at this time can be obtained as 77.59 kHz.  When the pump power increases from 47 mW to 515 mW, the pulse duration and repetition frequency change with the pump power as shown in Figure   10(a). It can be seen from the figure that when the pump power is 47 mW, the pulse duration is 13.85 μs, and the corresponding repetition frequency is 22.8 kHz. When the pump power is 515 mW, the pulse duration is reduced to 1.51 μs, and the repetition frequency is increased to 77.59 kHz. Obviously, the pump power is inversely proportional to the pulse duration and directly proportional to the repetition frequency. Figure 10(b) depicts the relationship between the output power and the single pulse energy with the pump power. When the pump power is 515 mW, the maximum average output power obtained is 3.58 mW, and the maximum single pulse energy is 46.14 nJ. It can be calculated that the light-light efficiency at this time is 0.7%, and the slope efficiency is 0.75%. In the experiment, the slope efficiency is relatively low. There are many reasons for this result, such as GeSe SA parameters, cavity design, OC split ratio and pump power limitation. Therefore, we can further optimize the parameters of the cavity and GeSe SA, and at the same time replace the OC, pump source and other experimental devices to improve the slope efficiency. Limited by the pump power in the experiment, the damage threshold of GeSe SA could not be obtained. Therefore, we infer that the use of a pump source with a larger pump power can achieve a higher repetition rate and further shorten the pulse duration, so that a better performance output pulse can be obtained.

Experiment and Results
Throughout the experiment, the fiber laser can obtain stable Q-switched pulses in the pump power range of 47 -515 mW. Figure    When the pump power is too high, the Q-switched operation will often disappear due to the pulse saturation. However, in our experiments, GeSe-based SA showed a higher damage threshold and good thermal stability until the pump power was increased to 515 mW. In order to verify the influence of GeSe SA on the erbium-doped fiber laser, we removed the fiber connector with GeSe-PVA film and replaced it with a clean fiber connector of the same length and model.
Adjusting the PC while changing the pump power, no matter how the adjustment is made, there is no Q-switched output pulse. Facts have proved that the Q-switched operation is realized in the fiber laser due to GeSe SA. Further comparison shows that GeSe nanosheets can be used as an excellent saturable absorber to achieve Q-switched pulse output. In the experiment, no optical damage  In order to find out the advantages and disadvantages of passive Q-switched operation based on GeSe SA; Table 1 shows the performance comparison of passive Q-switched erbium-doped fiber lasers based on various 2D materials.
The results show that, compared with the previous experimental materials, GeSe SA prepared by the LPE method has a higher modulation depth. In addition, the laser's repetition frequency, pulse duration, pulse energy and average output power also have obvious advantages.

Conclusion
In a word, this chapter proposes and proves the saturable absorption characteristics of GeSe nanosheets, using GeSe SA to achieve stable Q-switched pulse output in erbium-doped fiber lasers. In our experiments, the modulation depth MW/cm 2 , respectively. A ring cavity in the 1550 nm band was built, and the Q-switching operation was realized within the range of pump power from 47 mW to 515 mW, and the repetition frequency was increased from 22.8 kHz to 77.59 kHz. Among them, when the pump power is 515 mW, the maximum pulse output power of 3.58 mW is obtained, the shortest pulse duration is 1.51 μs, and the corresponding pulse energy is 46.14 nJ. Experimental results show that GeSe nanosheets can be used as high-efficiency SA in fiber lasers. During the whole experiment, the output of the system remained stable. Our results will provide useful references for demonstrating pulsed fiber lasers based on GeSe devices.