Layered metal dichalcogenides (LMDs) have been extensively employed as saturable absorbers (SAs) for demonstrating passively Q-switched or mode-locked lasers due to the advantages of wide absorption range and ultra-fast recovery time. In this paper, the nonlinear saturable absorption property of another IV-VI LMDs tin disulfide (SnS2) was investigated. SnS2-polyving alcohol (PVA) film was successfully prepared and employed as a SA for demonstrating an Er-doped passively Q-switched fiber laser. Under a pump power of 637 mW, the maximum average output power was 9.33 mW. The minimum pulse width was as narrow as 510 ns, which, to our knowledge, is the narrowest pulse width obtained within passively Q-switched Er-doped all-fiber ring-cavity lasers. The results indicate that film-type SnS2-PVA SA has excellent nonlinear absorption properties and outstanding performance in obtaining short-pulse passively Q-switched operation, which will promote the practical applications of SnS2 in the field of ultrafast photonics.
© 2017 Optical Society of America
Over the past decade, numerous reports have advanced the process of Er-doped passively Q-switched fiber lasers due to their wide applications in various fields, such as medicine, biomedical diagnostics, fiber optics sensing and material processing [1–6]. In comparison with actively Q-switched fiber lasers, passively Q-switched fiber lasers attracted much attention due to their advantages of simple all-fiber structure, low-cost and no requirement of additional drive device. It is widely known that nonlinear saturable absorber was a indispensable device of a passively Q-switched laser, the progress of saturable absorber materials determines the development of passively Q-switched lasers. Previously, semiconductor saturable absorber mirrors (SESAMs) [1-2], carbon nanotubes (CNTs) , graphene [4–6], graphene-like two dimensional materials (topological insulators (TIs) [7-8], black phosphorus (BP) , layered metal dichalcogenides [10–17]) have been used as SAs for demonstrating Er-doped passively Q-switched laser operations. In 1992, SESAM was successfully reported by U. Keller et al for the first time . Since then, commercialized SESAMs with controllable modulation depth and absorption wavelength have been widely reported. However, SESAM has some obvious weakness including narrow-absorption-bandwidth, high-cost, low damage threshold and complex preparation process, which have restricted its further in-deep development. CNTs have the advantages of low-cost and easily-preparation , thus, CNTs have been widely used in passively Q-switched lasers. However, its absorption efficiency and bandwidth were dependent on its diameters. So, SAs with wide absorption range and ultra-fast recovery time are still desired. The appearance of graphene have overcome the weakness of CNTs, Graphene has the advantages of wide absorption range, easy-preparation, low-cost, fast recovery time, high damage threshold and low saturation threshold. graphene based erbium-doped mode-locked fiber laser was first reported in 2010 . Since then, graphene is highly concerned as SAs for demonstrating pulse laser operations. But, the zero-bandgap structure limit its applications as optoelectronic devices. Inspired by graphene, two-dimensional materials including TIs, TMDs, and BP are extensively employed for demonstrating pulse laser operations. Especially, LMDs, such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2) [10–17], have shown significant feasibility in the sphere of non-linear optics and photo-electronics due to their properties of high three-order optical nonlinear susceptibility, ultra-fast carrier dynamics, and semi-conductive capabilities with a turnable bandgap. Recently, Q-switched Er-doped fiber lasers based on LMDs as SAs have been widely reported. Luo et al reported a 1.5 μm fiber laser Q-switched by a broadband few-layer MoS2 SA , the minimum single pulse width is 5.4 μs at the output power of 1.7 mW. Soon after, Woodward et al realized Q-switched Er-doped fiber lasers using MoSe2 as SA, which has a saturation intensity of 3.4 MW/cm2. The pulse duration of the laser fiber can be measured at 4.8 μs under the repetition rate of 35.4 kHz . Besides, WS2 was also used for demonstrating Q-switched lasers by Wu et al . In conclusion, LMDs are now under unceasingly emerging attentions on account of their excellent optical characteristics. In comparison with the mentioned LMDs (MoS2, MoSe2, WS2), studies on the application of other LMDs (SnS2, SnSe2, ReS2, ReSe2) in the field of ultra-fast pulse laser are still in its infancy. Exclusively, by employing the SnSe2-coated mirror as a saturable absorber, passively Q-switched waveguide laser at 1 μm was reported by Cheng et al . By using ReS2 as a saturable absorber, self-started mode-locking operation in an Er-doped fiber laser was reported by Cui et al in 2017 . In addition, A film-type ReS2-PVA based harmonic mode-locking operation in an erbium-doped fiber laser was also reported . In 2017, Mao et al reported an ReS2-polyvinyl alcohol (PVA) film based passively Q-switched and mode-locked erbium-doped fiber lasers . In comparison with the mentioned LMDs, SnS2 has attracted our attention due to its obvious advantages of low-cost, easily-prepared, suitable-bandgap, environmentally-friendly, earth-abundant and so on. SnS2 was a n-type direct bandgap semiconductor with a value of 2.24 eV and a CdI2 crystal structure [22-23], Previously, SnS2 have been widely relied to fulfill industrial and scientific requirements in the fields of solar cells, photocatalysts, and lithium ion batteries [22–25]. It is regarded as a admirable material which exhibits excellent electrical and optical properties and will promote the progress of low-cost photonic device. So, it is of great significance to expand the applications of SnS2 in different fields.
In this paper, SnS2 nanosheet was prepared and used as SA for demonstrating a passively Q-switched erbium-doped fiber laser. The physical form and nonlinear absorption properties of the SnS2 film were also examined experimentally. Stable passively Q-switched fiber laser operating at a central wavelength of 1532.7 nm was obtained. When the pump power increased from 290 to 637 mW, the repetition rate varies from 172.3 to 233.0 kHz, meanwhile, the pulse width decreases from 1.01 μs to 510 ns. Our results indicates that SnS2 with excellent nonlinear absorption properties has significant potential in obtaining stable passively Q-switched laser operation with short pulse width.
2. Preparation and characteristics of the SA material
In the experiment, SnS2 dispersion solution was prepared firstly. 1 g SnS2 nanosheets (shown in the insert of Fig. 1 (a)) was added into 100 ml alcohol (30%). Before that, based on a scanning electrical microscope (Sigma 500, ZEISS) with a resolution of 50 μm, the morphological characteristics of the SnS2 nanosheet was analyzed. As is shown in Fig. 1(a), SnS2 nanosheets with obvious lamellar structure was obtained. Figure 2(b) shows the EDX spectroscopy of the SnS2 nanosheet. As is shown, The corresponding peaks associated with sulfur and tin are clearly observed. Additionally, the surface distribution of elements was shown in the inset of Fig. 1(b). The results prove that pure SnS2 nanosheets with well layerd-structure was prepared.
the crystal structure of the SnS2 nanosheet was also studied by X-ray Diffraction (XRD) (D8 advance Bruker), as shown in Fig. 2(a). The XRD pattern exhibits high diffraction peaks at (001) plane which indicates that SnS2 nanosheets with well-layered structure were successfully prepared. In addition, the structural characterization of SnS2 was carried out by Raman spectroscopy (EnSpectr R532, EnSpectr, USA), whcih using a 532 nm exciting radiation of a 25 mW He–Ne laser and a scanning range from 100 to 750 cm−1. As is shown in Fig. 2(b), Raman spectrum with two Raman shift peaks at 314 and 205 cm−1, which corresponding to the to the A1g, and Eg modes of the SnS2, was observed. The results were in agreement with the results reported before . However, accurate Raman and XRD spectra are still not enough to estimate the specific layers of the SnS2 material. Therefore, transmission electron microscope (TEM) and atomic force microscope (AFM) will be used to determine the layered characteristics of material.
After that, the mixture was placed in the ultrasonic cleaner for 12 hours and then centrifuged at a rate of 2000 rpm for 30 minutes to remove the deposit. Then, the SnS2 dispersion and 4 wt% PVA solution were mixed at the volume ratio of 1:2, the mixture (shown in the insert of Fig. 3(a)) was placed in the ultrasonic cleaner for 4 hours to get the SnS2-PVA dispersion solution. For testing the layered structure properties of the SnS2, a JEM-2100 microscope with an optical resolution of 500 nm was employed. As is shown in Fig. 3(a), the SnS2 nanosheets exhibits obvious layered structure. Additionally, Fig. 3(b) shows the selected-area electron diffraction of the SnS2. The results indicate that the SnS2 nanosheets have obvious layered structure with high crystallinity. The thickness of SnS2 nanosheet was measured by an atomic force microscope (Bruker Multimode 8), as is shown in Fig. 4(a) and 4(b), the height difference between the substrate and the SnS2 sample surface is about 9.5 nm, corresponding to the layer number of about 11-12.
Afterwards, 100 μL SnS2-PVA dispersion solution was spin coated on a glass substrate. the coated substrate was placed into a oven for 48 hours at 30°C. Finally, a thin SnS2-PVA film was obtained. The optical transmission spectrum of the SnS2-PVA film was measured with a UV/vis/NIR spectrophotometer (Hitachi U-4100) and shown in Fig. 5. In addition, The transmission spectrum of the substrate was also shown as a comparison. It is obvious that the transmission increases with the wavelength. Finally, the transmission of the SnS2-PVA film at the wavelength of 1532 nm was about 78%.
The nonlinear absorption properties of the SnS2-PVA film-type SA were investigated by using a power-dependent transmission technique , the experimental setup is shown in Fig. 6, the pump source was a home-made nonlinear polarization rotation mode-locked Er-doped fiber laser with 560 fs pulses at 1580 nm with a repetition rate of 33.6 MHz. A variable optical attenuator (VOA) was used for power regulation.
3. Experimental setup
The experimental setup of the SnS2 based passively Q-switched Er-doped fiber laser is shown in Fig. 8. A ring laser cavity was employed. The pump source is a 980 nm laser diode (LD) with a maximum output power of 680 mW. The pump source injected into the ring laser cavity through a 980/1550nm wavelength division multiplexer (WDM). The gain medium was a 44 cm long erbium-doped fiber (EDF) with an absorption of 110 dB/m at 1530 nm and a dispersion value of about −46 ps/(nm.km) at 1550 nm. The polarization insensitive isolator (PI-ISO) was used to keep the laser unidirectional operation in the ring cavity. A polarization controller (PC) in the cavity was used to adjust the polarization state of the lase. The SnS2 saturable absorber was inserted between the PI-ISO and PC. A 10 dB output coupler (OC) was used to output the laser through its 10% port. The total cavity length is 5.09 m with a net dispersion of −0.081 ps2. The output performance of the fiber laser were record by a fast-speed photodetector (3G), a digital oscilloscope (DPO4054), a power meter (PM100D-S122C), a optical spectrum analyzer (AQ6317) and a spectrum analyzer (R&S FPC1000).
4. Results and discussion
In the experiment, unstable pulse laser operation was obtained at a pump power of 234 mW and the stable passively Q-switched pulses were attained at a pump of 290 mW. It is obvious that the threshold power of our work was much higher than the results reported previously. In our opinion, high threshold power was caused by the large intra-cavity loss which was mainly produced by the SA. The emission spectrum, which was detected by the optical spectrum analyzer with a resolution of 0.05 nm, is shown in Fig. 9(a), the central wavelength was located at 1532.7 nm with a 3 dB bandwidth of 0.03 nm. It is obvious that the operating photon energy of our work (0.81 eV) was much lower than the bandgap of SnS2 (2.24 eV), the results indicate that sub-bandgap absorption was responsibly for the passively Q-switched operation. Actually, sub-bandgap absorption phenomena have been widely reported before [27-28]. As is known, in a perfect crystal, there is no sub-bandgap absorption. However, in a finite system, the sub-bandgap absorption at low photon energies could also be realized which attributed to energy levels within the bandgap arising from edge-state. In our opinion, sub-bandgap absorption observed in our work was also attributed to the edge-state absorption of the SnS2. The relationships between the average output power and the pump power is shown in Fig. 9(b), It is obvious that there is a clear linear relationship between the pump power and the average output power. The maximum average output power was as high as 9.33 mW under a pump power of 637 mW, corresponding to an optical-to-optical conversion efficiency of 1.46%. Finally, based on an optical power meter. the total cavity loss of the SnS2-PVA device is measured to be about 6.1 dB, whcih indicates that the low output power and optical-to-optical conversion efficiency were also mainly due to the large intra-cavity loss produced by the SA.
In the experiment, when the power varies between 290 and 637 mW, passively Q-switched operation was stable. however, when the power is higher than 645 mW, the Q-switched operation became unstable, but when the power was adjusted to below 645 mW, the operation became stable again, indicating that the instability of the pulse operation was caused by the saturated absorption of the saturable absorber. However, when we added the pump power to 660 mW and kept for ten minutes and then reduced to less than 645 mW, no passively Q-switched phenomenon was recorded, which means that the saturable absorber has been damaged. Therefore, the damage threshold of the SA was about 660 mW. In order to protect the SA, in the experiment, we use the maximum pump power no more than 640 mW.
Figure 10(a) shows the pulse widths and repetition rates versus different pump powers. As is shown, when the pump power increasing from 290 mW to 637 mW, the pulse width varies from 1.01 µs to 0.51 µs. Meanwhile, the repetition rate increases from 172.3 kHz to 233 kHz, which are typical features of passively Q-switched fiber lasers. The corresponding pulse energies and peak powers under different pump power were shown in Fig. 10(b), as is shown, the pulse energy and peak power all increased with the growth of the pump power. Figure 11 shows the typical Q-switched pulse trains and their corresponding single pulse profile under different pump power of 290, 489 and 637 mW.
The stability of the SnS2-PVA based passively Q-switched fiber laser under the maximum pump power was tested by a spectrum analyzer (R&S FPC1000), Fig. 12 depicts the radio frequency spectrum located at the fundamental repetition rate of 233 kHz with a signal-to-noise ratio is about 50 dB. In addition, the radio frequency spectrum within a wide bandwidth of 3 MHz is shown in Fig. 12(b). The results of the radio frequency spectrum all prove that SnS2-PVA based passively Q-switched fiber laser with high stability was obtained in our work.
As is mentioned, based on different SAs, passively Q-switched Er-doped all-fiber ring-cavity lasers have been widely reported. Table 1 shows relatively complete parameters of previous reported passively Q-switched Er-doped all-fiber ring-cavity lasers which based on different materials as saturable absorbers. It should be noted that the results of passively Q-switched Er-doped all-fiber linear-cavity lasers have not been included due to the difference between the laser resonators. As is shown, based on a ring-cavity demonstration, 510 ns pulse width obtained in our work is the narrowest of all. Additionally, based on LMDs as SAs [7,16], the maximum output power was also obtained in our works, indicating that SnS2 has significant advantages in obtaining high-power short pulse laser over the LMDs reported before. Therefore, it is of great significance to make an intensive study of SnS2 in the field of photonics.
In conclusion, SnS2 was demonstrated as a new SA for obtaining stable passively Q-switched erbium-droped fiber laser. The maximum output power was 9.33 mW under a pump power of 637 mW. The narrowest pulse width was as short as 510 ns. Our experimental results clearly proved that film-type SnS2-PVA SA has excellent nonlinear abrorption properties and amazing performance in obtaining short-pulse passively Q-switched operation.
Shandong Provincial Natural Science Foundation (ZR2016FP01, ZR2014FM028); China Postdoctoral Science Foundation (2016M602177); National Natural Science Foundation of China (nos.61475089, 61205174, 11474187).
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