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Abstract
In our work, Bi2Se3 was successfully used for demonstrating a large-energy passively Q-switched erbium-doped fiber laser. Bi2Se3 nanosheets were fabricated by a catalyst-free chemical vapor deposition method. Based on a pyrolysis tape transfer method, the Bi2Se3 thin film on the SiO2 substrate was transferred to the end of the optical connector for constructing the fiber-integrated saturable absorber. The saturation intensity and modulation depth of the Bi2Se3 saturable absorber were 81.1 MW/cm2 and 15.7%, respectively. A stable Q-switched operation at 1549.99 nm with a maximum average output power of 23.61 mW was achieved. The minimum pulse duration and the largest pulse energy were 1.34 μs and 224.5 nJ, respectively. In comparison with previous works, the single pulse energy (224.5 nJ) obtained in our experiment was improved significantly. Our experimental results fully proved that CVD-Bi2Se3 has good performance in obtaining large energy pulse operations and will promote the applications of 2D CVD-materials in the field of pulse laser.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Large-energy Q-switched fiber lasers have attracted much attention due to their wide applications in the fields of remote sensing, material processing, medicine, communication and so on [1–16]. Compared to actively Q-switched fiber lasers, passively Q-switched fiber lasers possess the advantages of flexibility, compactness and simplicity [1–13]. Previously, passively Q-switched Er-doped fiber lasers have been investigated intensively by employing different kinds of saturable absorbers (SAs), including semiconductor saturable absorber mirrors (SESAMs) [1], carbon nanotubes (CNTs) [3], graphene [6,7], layered metal dichalcogenides (LMDs) [10–16], black phosphorus (BP) [17,18], topological insulators (TIs) [19–22,34,35], and so on. SESAMs have the advantages of controllable saturation intensity and modulation depth, however, its also have the disadvantages of narrow absorption wavelength range, complex fabrication process and high cost [1,2]. CNTs were easily-prepared with tunable absorption peak and low cost. CNTs-based fiber lasers always exhibit high pump threshold due to the scattering of different-diameter CNTs [3–5]. Graphene has overcome the weaknesses of SESAMs and CNTs. Graphene has the advantages of wide absorption range, easy preparation, low cost, fast recovery time, high damage threshold and low saturation threshold [6–9]. But, its zero-bandgap structure also limited its applications in the field of opto-electronic devices. Besides, graphene-like two-dimensional materials including TIs, TMDs, and BPs have also been extensively employed for demonstrating pulse laser operations due to their properties of suitable band-gap values, ultra-fast recovery time, wide absorption range, high damage threshold and so on [10–23,34–36]. In brief, two-dimensional materials promoted the rapid development of pulsed fiber lasers significantly. Especially, TIs have been confirmed to exhibit unique photonics properties owing to its broadband spectral response ranging from terahertz to infrared, which was due to its intrinsic gapless surface band. Bismuth selenide (Bi2Se3), as a typical topological insulator, has attracted much attention in recent years due to its advantages of low saturation intensity, wide nonlinear absorption band and high damage threshold. And it has the most prominent advantage of a relatively large bulk band gap (0.3 eV), which corresponding to a wide absorption wavelength (about 4130 nm). It was first used as a saturable absorber in Er-doped fiber laser in 2012 [26]. In addition, femtosecond pulse generations were obtained by using Bi2Se3 as saturable absorbers [27,28]. In 2015, based on the interaction between the Bi2Se3 and photonics crystal fiber, Gao et al also demonstrated a conventional soliton mode-locked operation with a 908 fs pulse width and a dissipative soliton with a duration of 7.56 ps [24]. Besides, Guo et al have experimentally demonstrated several different Bi2Se3-based mode-locked generations including dual-wavelength harmonic, dark soliton and multi-wavelength within Er-doped fiber lasers [29–31]. Other different soliton operations such as bound-soliton states [25], harmonic [32] and dissipative rogue waves [33] have also been observed within Bi2Se3-based mode-locked fiber lasers. Besides the mentioned mode-locked operations, Y. Z. Huang et al demonstrated a Bi2Se3-based passively Q-switched linear-cavity Er-doped fiber laser, the used Bi2Se3 nanosheets were prepared by the liquid-phase exfoliation technique, and the obtained pulse energy was 2.38 nJ under a pulse repetition rate of 62.7 kHz [34]. Z. H. Yu et al also demonstrated a high-repetition-rate Bi2Se3-based Q-switched fiber laser, the maximum average output power and the largest pulse energy were 22.35 mW and 23.7 nJ, respectively [35]. All results have already proved that Bi2Se3 exhibited excellent optical and nonlinear saturable absorption properties, which ensured it to be a suitable saturable absorber for demonstrating ultra-fast laser operations. However, the Bi2Se3 nanosheets, which used as SAs in the mentioned works, were all prepared by liquid-phase exfoliation methods. As is known, liquid-phase exfoliation method has the advantages of low-cost, easy-preparation and so on. However, the stripping efficiency is relatively low. In addition, the morphology and saturable absorption parameters of the prepared Bi2Se3 nanosheets are uncontrollable. Chemical vapor deposition (CVD) is another efficient method which has been widely employed for preparing 2D materials. Compared with other preparation methods including liquid phase epitaxy, mechanical exfoliation, liquid-phase exfoliation, pulse laser deposition, magnetron sputtering deposition and so on, CVD method exhibits the advantages of simple equipment, easy operation and low cost. And, 2D materials prepared by CVD method have the most important characteristics of uniform shapes and controllable layer numbers [36,37]. As is known, The most important parameters for 2D material-based SAs were saturation intensity and modulation depth, which depended on the layer numbers. In conclusion, CVD method will improve the controllability of the nonlinear absorption parameters of the SAs and promote the applications of 2D materials in the field of ultra-fast optics.
In our work, CVD-Bi2Se3 with good uniformity, high damage threshold, and large modulation depth were successfully prepared, and employed as SA for demonstrating large-energy passively Q-switched operation. Within a linear-cavity, stable passively Q-switched operation with a maximum average output power of 23.61 mW under the pulse repetition rate of 105.15 kHz was achieved, corresponding to a pulse energy of 224.5 nJ. Our results proved that CVD-Bi2Se3 have significant advantages in demonstrating high power and large-energy pulse operations.
2. Preparation and characterization of the CVD-Bi2Se3 SA
In our experiment, Bi2Se3 nanosheets were prepared by a commonly reported catalyst-free CVD method [36,37]. Two-steps preparation process was carried out in a horizontal tube furnace (OTL1200). Firstly, a thin layer of Se film was pre-deposited on a SiO2 substrates. At first, SiO2 substrates were cleaned in acetone, ethanol, and deionized water, respectively. And then, Se powder was employed as an evaporation source in the constant-temperature horizontal tube furnace. The substrates was located about 15 cm away from the evaporation source. After that, the tube was pumped into vacuum and heated to the growth temperature of 200 °C under Ar flow rate of 50 sccm. The growth temperature was kept for 30 minutes. Finally, the Se crystalline grains were successfully synthesized on the SiO2 substrates. The second step is to synthesize the Bi2Se3 thin films on the Se/SiO2 substrates based on CVD method. Bi2Se3 powder was also employed as an evaporation source. Meanwhile, the Se/SiO2 substrates was placed downstream away from the Bi2Se3 source with a distance of about 17 cm. In the experiment, by using ultra-pure Ar as carrier gas, Bi2Se3 vapor was brought to the Se/SiO2 substrates under a temperature of 550 °C in a vacuum environment. As is known, the melting point of Se was 221 °C and the volatilization of Se would lead to the the formation of Se vacancies and irregular deposition of Bi2Se3. Thus, the temperature of the furnace was firstly elevated to be 550 °C. And then, the tube with Bi2Se3 powder and Se/SiO2 substrates was moved into the furnace. Finally, Bi2Se3 thin films were successfully synthesized under constant temperature of 550 °C for 18 min. At last, the furnace was rapidly cooled down to ambient temperature under Ar flow rate of 50 sccm.
For testing the layered-structure and shape characteristics of the CVD-Bi2Se3 nanosheets, a scanning electron microscope (SEM) (Sigma 500) was employed, SEM images of the CVD-Bi2Se3, which were recorded under different resolutions, are shown in Fig. 1(a) and (b), respectively. As is shown, Bi2Se3 nanosheets exhibit uniform shape and obvious typical layered structure. The structure characterizations of the Bi2Se3 nanosheets were also investigated with a Raman spectrometer (Horiba HR Evolution) and a X-ray Diffraction (XRD) (D8 advance Bruker). Obvious Raman shifts at 73.7 cm−1(A1g1), 132.4 cm−1 (Eg2), and 175.2 cm−1 (A1g2) were recorded and shown in Fig. 1(c), the results were in agreement with the previous reported works [36,37]. No other Raman shifts were observed in the spectrum, indicating that only Bi2Se3 was prepared in our work. The diffraction XRD spectrum of the Bi2Se3 is shown in Fig. 1(d), as is shown, peaks corresponding to the (003), (006), (009), (0012), (0015), (0018) and (0021) planes in Bi2Se3 were recorded. Especially, the high diffraction peak at the (006) plane indicates that the nanosheets exhibit well-layered structure and high crystallinity.
Fig. 1 (a) The SEM image of the Bi2Se3 nanosheets with the scale of 1000 nm. (b) The SEM image of the Bi2Se3 nanosheets with the scale of 200 nm. (c) The Raman spectrum of the Bi2Se3 nanosheets. (d) The X-ray Diffraction of the Bi2Se3 nanosheets.
Meanwhile, the energy dispersive X-ray spectroscopy (EDX) of the prepared Bi2Se3 nanosheets was also recorded by the mentioned SEM. As is shown in Fig. 2(b), peaks corresponding to the Bi and Se were recorded. The atomic ratio of Se to Bi is 1.69, which is large than the stoichiometric ratio of 1.5 for Bi2Se3 due to the impact of other atoms [36,37].
The thickness characteristics of the prepared CVD-Bi2Se3 nanosheets were tested by an atomic force microscope (AFM) (Bruker Multimode 8), as is shown in Fig. 3(a) and (b), the thicknesses of the marked sample is about 100 nm, corresponding to the layer numbers of about 100. In addition, as is depicted in Fig. 3(b), the prepared nanosheets exhibit uniform flatness.
For proposing commonly used sandwich-structure Q-switcher, The Bi2Se3 thin films on the SiO2 substrate (shown in Fig. 4(a)) were transferred to the end of the optical connector based on a pyrolysis tape transfer method. To begin with, the pyrolysis tape was pasted on the Bi2Se3 thin film on the SiO2 substrate. And then, the pyrolysis tape was removed from the substrate (shown in Fig. 4(b)). Meanwhile, pyrolysis tape was bonded to each other for three times and pasted to the fiber end-face. Ultimately, the fiber connector with the pyrolysis tape (shown in Fig. 4(c)) was placed on the heating platform (130 °C) for 5 min. Finally, with the disappearance of the adhesive force of the pyrolysis tape, thin Bi2Se3 film was successfully transferred to the end of the optical connector for proposing a Q-switcher .
Fig. 4 (a) The image of the Bi2Se3 on SiO2 substrate. (b) The image of the Bi2Se3 nanosheets on pyrolysis tape. (c) The image of the optical connector with the pyrolysis tape.
The nonlinear optical saturable absorption response of the Bi2Se3 saturable absorber was investigated experimentally based on the same setup and method which were reported in our previous work [38–40]. The pump source was a home-made nonlinear polarization rotation mode-locked Er-doped fiber laser, the central wavelength, pulse width and pulse repetition rate were 1580 nm, 560 fs and 33.6 MHz, respectively [40]. The experiment results of the relationship between the transmission and the optical intensity is shown in Fig. 5. Additionally, based on the well-known formula [38,39]:
where T is transmission, Tns is non-saturable absorbance, ΔT is modulation depth, I is input intensity of laser, Isat is saturation intensity, the saturation intensity and modulation depth of the Bi2Se3 Q-switcher, which were obtained by fitting the experiment results, were 81.1 MW/cm2 and 15.7%, respectively. It should be noted that, as is shown in Fig. 5, the nonsaturable loss was as high as about 46%, which was relatively large, in our opinion, the large loss of the SA was mainly due to the hundred-level layers of the Bi2Se3 nanosheets.3. Experimental setup of a Bi2Se3-based passively Q-switched Er-doped fiber laser
In our experiment, a simple linear laser cavity was demonstrated for achieving large-energy pulse operations. In comparison with the ring laser cavity which was commonly used for demonstrating Q-switch and mode-locked fiber laser operations, linear laser cavity has advantages in obtaining passively Q-switched operations with large energy and short pulse width. The Bi2Se3-based passively Q-switched linear-cavity Er-doped fiber laser is schematically depicted in Fig. 6. The pump source is a 980 nm laser diode (LD) with a maximum output power of 530 mW. The pump source was injected into the linear laser cavity through a 980/1550 nm wavelength division multiplexer (WDM). A 31 cm long erbium-doped fiber (Liekki, Er110, 4/125) with a dispersion value of about −46 ps/(nm.km) was used as laser gain medium. A pair of fiber Bragg gratings (FBGs) was employed for proposing the laser cavity. FBG1 has a high reflectivity of >99.6% at the central wavelength of 1549.96 nm with a 3 dB width of 0.36 nm, FBG2 has a reflectivity of 90% at the central wavelength of 1549.93 nm with a 3dB width of 0.33 nm. Additionally, the CVD-Bi2Se3 SA was inserted into the cavity between the gain medium and FBG2. The total cavity length was about 1.37 m. The output characteristics of the passively Q-switched fiber laser were recorded by a fast-speed InGaAs photodetector (3G), a digital oscilloscope (Tektronix, DPO4054), a power meter (Thorlabs, PM100D-S122C), a optical spectrum analyzer (Yokogawa, AQ6317), respectively.
4. Experimental results and discussions
Firstly, without inserting the SA into the laser cavity, only continuous wave operation was observed by adjusting the pump power. In contrast, once we added the SA into the linear cavity, stable Q-switched operation was recorded when the pump power exceeded to be 80 mW, which indicating that the Bi2Se3 SA was responsible for the passively Q-switched operation. Meanwhile, by adjusting the pump power, no mode-locked operations were recorded in our experiment, which was mainly due to the short-length of the linear-cavity. Because the formation of mode-locked pulse was the result of the dynamic balance between total cavity dispersion, laser gain, total cavity loss and a variety of nonlinear effects, which were easily obtained within long-length ring laser cavity. Figure 7 shows different typical Q-switched pulse trains under different pump powers from 80 to 530 mW. As is shown, stable pulse generations were successfully achieved under different pump powers. Due to the limitation of the pump power, the maximum pump power was 530 mW. However, in our opinion, stable pulse generations should be always obtained under higher pump power. In our experiment, for testing the damage threshold of the CVD-Bi2Se3 SA, the pump power was set at 530 mW for 30 minutes, stable passively Q-switched operation was recorded all the time and the SA was not damaged under the pump power of 530 mW, indicating that the damage threshold power of the CVD-Bi2Se3 SA was higher than 530 mW. However, due to the limitation of the maximum output power, the actual damage threshold of the SA was not measured.
Fig. 7 Typical oscilloscope traces of the Q-switched pulse trains under different pump powers: (a) 80 mW, (b) 220 mW, (c) 370 mW and (d) 529.3 mW.
Figure 8 shows typical characteristics of the Q-switched operation under the pump power of 530 mW. Figure 8(a) shows the emission spectrum, which was recorded by the optical spectrum analyzer with a resolution of 0.05 nm. The central wavelength was 1549.99 nm with a 3 dB bandwidth of 0.048 nm, which was in agreement with the parameters of the FBGs. Single typical pulse shape with the pulse width of 1.34 µs is shown in Fig. 8(b). To our knowledge, 1.34 µs is the narrowest pulse width obtained within Bi2Se3-based passively Q-switched Er-doped fiber lasers.
Fig. 8 (a) The emission spectrum of the fiber laser at a pump power of 530 mW. (b) typical single pulse shape.
The characteristics of the pulse width and pulse repetition rate under different pump power were also investigated in the experiment. The variation curves are shown in Fig. 9(a). As is shown, when the pump power increases from 80 to 530 mW, the pulse width varies from 19.56 µs to 1.34 µs. Meanwhile, the repetition rate increases from 10.0 kHz to 110.2 kHz, which are all typical features of a passively Q-switched fiber laser. The relationships between the average output power, single pulse energy and the pump power are shown in Fig. 9(b), the average output power increases linearly with the increase of the pump power. The maximum average output power is as high as 23.61 mW under the pump power of 530 mW, corresponding to an optical to-optical conversion efficiency of 4.46%. Besides, the pulse energy increases with the growth of the pump power and the maximum single pulse energy is as high as 224.5 nJ.
Fig. 9 (a) pulse repetition rate and duration as a function of incident pump power. (b) Average output power and single pulse energy as a function of the pump power.
As is mentioned that various kinds of SAs have been successfully employed for demonstrating passively Q-switched fiber lasers. The output performance of passively Q-switched fiber lasers can fully present the characteristics of the SA materials. Tab.1 shows relatively complete parameters of previous reported passively Q-switched Er-doped fiber lasers which based on different materials as SAs. As is shown, previously, based on CVD-materials as SAs [7,12], the pulse widths were 3.2 and 1.66 μs, the 1.34 μs, respectively. The pulse width obtained in our work is relatively narrow. Additionally, the maximum average output power was also obtained in our work. Recently, based on BP-PMMA film as SA, passively Q-switched Er-doped fiber laser operation with a maximum pulse energy of 283.91 nJ was obtained [18]. Besides, based on the laser deposition method, a Bi2Te3 based passively Q-switched operation with a maximum pulse energy of 1.525 μJ was also reported [22]. In addition, in comparison with other reported works [1,3,6,7,10–17,19–21,23,34,35], the pulse energy obtained in our work was also improved significantly. In conclusion, the output performance of our work exhibits absolute improvement in comparison with previous works. Especially, in comparison with the reported Bi2Se3 SAs [23,34,35], the saturable absorption parameters (modulation depth and saturation intensity) of our sample were all larger. All the mentioned results indicated that CVD-Bi2Se3 has significant advantages in obtaining high-power, large-energy, short-pulse lasers over the SAs reported before, because, as is mentioned that CVD-Bi2Se3 exhibits higher damage threshold, controllable nonlinear saturable absorption parameters and layered-construction.
Table 1. Comparison of passively Q-switched Er-doped lasers based on different SAs.
In summary, a large-energy passively Q-switched Er-doped fiber laser was demonstrated. Bi2Se3 nanosheets, prepared by chemical vapor deposition method, with a saturation intensity and a modulation depth of 81.1 MW/cm2 and 15.7% was successfully employed as SA. Stable Q-switched laser emission at 1549.99 nm with a maximum average output power of 23.61 mW was generated, the shortest pulse width was 1.34 µs and the maximum single pulse energy was 224.5 nJ. In comparison with previous works, the output performance of our work have been improved significantly. Our results indicate that the 2D materials prepared based on CVD method have good optical properties and obvious advantages in demonstrating high-power, large-energy and short-pulse laser generations. Thus, in our opinion, our results will promote the applications of 2D CVD-materials in the field of pulse laser.
Funding
National Science Foundation (NSF) (11474187, 11747149).
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