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Abstract
We demonstrated a passively mode-locked Er-doped fiber laser by a high-efficiency Bismuth Telluride (Bi2Te3) as passive saturable absorber. The Bi2Te3 thin film was grown by chemical vapor deposition (CVD). The maximum average output power and pulse energy were as high as 40.74 mW and 23.9 nJ under the pump power of 659 mW, which are much higher than the results obtained previously. The signal to noise ratio was observed to be more than 60 dB, which indicates the stability of the generated pulse. Our results proved that CVD-Bi2Te3 was an excellent candidate for demonstrating large-energy pulse operations on mode-locked Er-doped fiber laser.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the past decade, passively mode-locked fiber lasers have gained great attention due to their wide applications in various fields, such as medicine, nonlinear optics, biomedical diagnostics, laser spectroscopy, and material micromachining [1–8]. Especially, in the field of nonlinear optics, the mode-locked fiber laser serves as an ideal platform and a powerful experimental tool for studying various nonlinear phenomena and revealing their versatile dynamics. Graphene, as the progenitor of two-dimensional materials, has been extensively employed as saturable absorbers (SAs) for demonstrating pulse laser operations due to its advantages of wide absorption range, easy-preparation, low-cost, fast recovery time, high damage threshold and low saturation threshold. The application of graphene has triggered a great mass fervour in research interest in other 2D nanomaterials, which have a turnable bandgap remedying the limitation of graphene’s zero-bandgap structure. Recently, in the efforts of novel two-dimensional (2D) materials including graphene [9–14], carbon nanotubes [15,16], topological insulators (TIs, Bi2Te3, Bi2Se3, Sb2Te3) [17–19], transition metal dichalcogenides (TMDs) [20–24], and black phosphorus (BP) [25–27], researches on various nonlinear phenomena observed in passively mode-locked fiber ring cavity have attracted a certain attention.
In those SAs mentioned above, topological insulators (TIs), a rising Dirac material, exhibit Dirac-like linear band dispersion [28], the ultra large saturable intensity and modulation depth [29–34]. Noteworthy, TIs also possess broadband wavelength operation characteristic and giant third order nonlinear property [35,36]. Therefore, TIs have attracted much attention for generating short and high-energy pulses [37–40], and a number of reports on TI-based fiber lasers appeared. A harmonically mode-locked Er-doped fiber laser with the output power of 4.5 mW and the pulse energy of 14.8 pJ by using the Sb2Te3 as SA was reported by J. Sotor [41]. And then, another all-fiber Er-doped fiber (EDF) laser mode-locked by Sb2Te3 SA has been presented, and the maximum output power is 5.34 mW [42]. Shuqing Chen et al. demonstrated a single-longitudinal-mode (SLM) fiber ring laser based on Bi2Te3 SA, generating the pulse which output power up to 23 mW [43]. While for those compact all-fiber lasers based on topological insulators saturable absorbers (TISAs) reported, the disadvantages of them are the low average output power. The main limiting factor of the output power is the lower damage threshold of topological insulators. Thus, the compact fiber lasers with high output power, high pulse energy, high transfer efficiency, low mode-locking threshold and narrow pulse duration are urgently needed through finding effective ways to increase the optical damage power of a TISA .
The fabrication methods of TI materials can be generally classified by these methods which are widely used for laser applications. They are mechanical exfoliation (ME) [44], solution processing (including Lithium ion intercalation exfoliation and liquid-phase exfoliation), Molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) [45]. Compared with other mothods, the TI materials produced by CVD possess the advantages of uniform shapes, controllable layer numbers, high damage threshold power and so on [46,47]. Therefore, CVD method has become the main method to synthesize TI materials.
In this paper, an Er-doped mode-locked fiber laser based a Bi2Te3 SA by CVD method for demonstrating efficient 1.5 µm fiber pulse generation was employed. Compared with the results previously reported, the average output power and pulse energy obtained in our work have been significantly enhanced. To our knowledge, this is also the first demonstration of an all-fiber structure EDF laser operating in the 1.5 µm region based on CVD-Bi2Te3 SA. We studied its appearance, linear transmission and nonlinear saturation absorption characteristics, where the transmission of the Bi2Te3 film at the wavelength of 1558 nm was 86.5%, the saturation intensity and modulation depth were about 5.8 MW/cm2 and 10.8%, respectively. Based on the Bi2Te3 film as SA, a mode-locked Er-doped fiber laser operating at the central wavelength of 1558.418 nm with a 3 dB bandwidth of 1.696 nm was demonstrated under the pump power of 115 mW. When the pump power increased from 372 to 659 mW, the output power varies from 20.4 to 40.74 mW, meanwhile, the maximum pulse energy is 23.9 nJ. Our experimental results proved that Bi2Te3 had excellent nonlinear saturation absorption characteristics and was an excellent mode-locker for achieving large pulse energy operations in the field of ultrafast optics.
2. Preparation and characterization of materials
In the experiment, the Bi2Te3 thin films were synthesized by a CVD method, as shown in Fig. 1(a). First, in order to clean the surface impurities, the SiO2 substrates were ultrasonically cleaned in acetone, ethanol, and deionized water respectively. Then, Bi2Te3 powder was placed in the constant-temperature zone of the horizontal tube furnace as an evaporation source. The substrates were located approximately 22 cm away from the evaporation source. During the process, the mechanical pump worked in vacuum mode (10−3Pa), and the tube was heated to the growth temperature of 530℃ under Ar (purity of 99.99%) of 30 sccm. The growth temperature was maintained for 30 min to synthesize the nanofilm. Finally, when the whole process was over, the furnace was rapidly cooled down to the room temperature, Bi2Te3 thin films were successfully prepared.
Fig. 1. (a) Schematic diagram for the growth of Bi2Te3 by CVD method, (b) Preparation process of the Bi2Te3 SA.
For obtaining commonly used sandwich-structure SA, the Bi2Te3 thin films on the SiO2 substrate were transferred to the end of the optical connector based on the pyrolysis tape transfer method in our experiment, as shown in Fig. 1(b). First, the pyrolysis tape was pasted onto the Bi2Te3 thin film on the SiO2 substrate. Then, the pyrolysis tape was removed from the substrate. Meanwhile, the film and the pyrolysis tape were bonded together and pasted onto the entire fiber end-facet. The fiber connector with the pyrolysis tape was then placed on the heating platform (130℃) for 10 min. Finally, after the adhesive force of the pyrolysis adhesive tape disappeared, the thin Bi2Te3 film was successfully transferred to the end of the optical connector and connected to the laser path for a saturated absorber.
The surface topography of the Bi2Te3 nanosheets was analyzed by a scanning electron microscope (SEM). Figure 2(a) shows the SEM image under a resolution of 1 µm and 200 nm, respectively, where we can see that the Bi2Te3 nanosheet has an obvious layered structure. Figure 2(b) shows the Energy Dispersive Spectrometer (EDS) spectrum of the Bi2Te3 nanofilm. The peaks associated with bismuth and tellurium are clearly observed. Besides, the Si peak and the O peak are derived from the SiO2 substrate, and the Au peak is caused by the “gold plating” process during the test. The atomic ratio of the Bi and Te elements is shown in the Fig. 2(b) inset, and the Te:Bi atomic ratio of the sample is 1.545.
Fig. 2. (a) The SEM images of Bi2Te3, (b) EDS spectrogram of the Bi2Te3 film. Insert of (b) the atom ratio, (c) Topographic AFM images of the Bi2Te3 nanosheets, (d) Corresponding height profiles, (e) The Raman spectrum of the Bi2Te3, (f) The XRD spectrum of the Bi2Te3 nanosheets.
To further confirm the thickness and the width of as-prepared Bi2Te3 nanosheets, the morphology of Bi2Te3 was investigated with atomic force microscope (AFM). As can be seen clearly from the Fig. 2(c), multilayer Bi2Te3 nanosheets are synthesized on substrates. As shown in Fig. 2(d), it can be seen that the thickness of nanosheets is about 60 nm.
The Raman spectrum of the layered Bi2Te3 is shown in Fig. 2(e). Apparently, four Raman shift peaks, corresponding to the A11g , E2g , A1u and A21g symmetry intralayer mode, at 57.897, 97.89, 115.47 and 135.063 cm−1, were characterized. Among them, E2g belongs to the in-plane vibration mode, meanwhile, A11g, A1u and A21g belong to the out-of-plane vibration mode [48,49]. The A1u mode appearing in the Raman spectrum indicates the ultrathin properties of the two-dimensional nanomaterials of the antimony telluride sample material. The crystal structure of the Bi2Te3 nanosheets was characterized by X-ray diffraction (XRD). As shown in Fig. 2(f), the XRD pattern exhibited high [006, 0015] orientations, which indicates that Bi2Te3 nanosheets with a welllayered structure and high crystallinity were successfully prepared.
The linear transmission of the Bi2Te3 film on substrate and the substrate versus optical wavelength were measured with a UV/Vis/NIR spectrophotometer (Hitachi U-4100) shown in Fig. 3(a). In addition, it is obvious that the transmission increases with the wavelength. The transmission of the Bi2Te3 film at the wavelength of 1558 nm was about 86.5% according to the transmittance curve shown in Fig. 3(a).
Fig. 3. (a) Linear transmission of the Bi2Te3 film on substrate and the substrate versus wavelength, (b) Nonlinear absorption property of the Bi2Te3 film.
The nonlinear absorption properties of the Bi2Te3 film-type SA were investigated by using a power-dependent transmission technique [50]. A homemade nonlinear polarization rotation mode-locked Er-doped fiber laser with 560 fs pulses at 1560.3 nm and a repetition rate of 33.6 MHz, the nonlinear optical properties of the Bi2Te3 film were investigated. As is shown in Fig. 3(b), based on the following formula [51]:
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 were 5.8 MW/ cm2 and 10.8% by fitting the experimental results.In addition, the Bi2Te3-SA transmittance from Fig. 3(a) and Fig. 3(b) is significant difference. The discrepancy is mainly due to the different pump source. The linear transmittance in Fig. 3(a) was tested by employing a continuous-wave laser as a pump source, meanwhile, a picosecond pulsed laser was used for testing nonlinear absorption property of the Bi2Te3-SA.
3. Experimental setup
Figure 4 shows the experimental setup of the Bi2Te3 based Er-doped fiber laser. A 980 nm laser diode (LD) with a maximum output power of 1250 mW was used as the pump source. The pump source was injected into the ring laser cavity through a 980/1550 nm wavelength division multiplexer (WDM). A piece of 62 cm long Er-doped fiber (Er-80, 8/125) with a dispersion parameter of∼15.7 ps/(nm·km) was employed as the laser gain medium. A 60:40 optical coupler (OC) was used to output the laser. The polarization insensitive isolator (PI-ISO) was used to keep the laser unidirectional operation in the ring cavity. Two polarization controllers (PC) in the cavity were used to adjust the polarization state of the laser. The Bi2Te3 saturable absorber was inserted between the SMF and PC. The total length of the cavity was about 121 m [120.38 m single-mode fiber (SMF-28) with a dispersion value of about 17 ps/(nm·km) ]. Thus, the net dispersion of the Er-doped fiber laser was calculated to be −2.73 ps2.
The output characteristics of the mode-locked laser were record by a fast-speed InGaAs photodetector (3G), digital oscilloscope (DPO4054), power meter (PM100D-S122C), optical spectrum analyzer (AQ6317) and spectrum analyzer (R&S FPC1000).
4. Results and discussion
In the experiment, when the pump power was 115 mW, by carefully adjusting the PCs in the cavity, stable pulse trains can be recorded. The uniform shape of the output pulse train in time domain is shown in Fig. 5(a) depicting less fluctuation from peak to peak with the pulse repetition rate of 586.8 ns, corresponding to fundamental frequency mode-locked of the 121 m long laser cavity. The output pulse with the duration of 3.22 ns is shown in Fig. 5(b). The emission spectrum of the Bi2Te3-based Er-doped fiber laser, which was recorded by an optical spectrum analyzer is shown in Fig. 5(c). As is shown, typical soliton-like pulse shapes with characteristic Kelly sideband peaks were obtained. Additionally, stability is one of the most important parameters that restricts the practical application of the mode-locked laser. For testing the stability of the Bi2Te3 based mode-locked laser, the radio frequency spectrum was recorded by a spectrum analyzer (R&S FPC1000). The RF spectra in Fig. 5(d) show that signal/noise ratio (SNR) is about 60 dB and the fundamental repetition rate of 1.704 MHz, which was determined by the cavity length with the resolution bandwidth(RBW) set as 300 Hz and the video bandwidth (VBW) set as 300 Hz. The inset of Fig. 5(d) performs the RF spectrum in a 170 MHz range with 1 kHz RBW and 1 kHz VBW. All the results exhibited that mode-locked pulses with high stability were obtained in our work.
Fig. 5. (a) Optical pulse train of the mode-locked operation, (b) Single pulse with a pulse width of 3.22 ns, (c) The output spectrum with 3 dB bandwidth of 1.696 nm, (d) The RF spectra of the optical pulse.
The average output power and the pulse energy as a function of the pump power are shown in Fig. 6(a). As shown, the pulse energy changes from 11.97 to 23.9 nJ and the output power increases from 20.4 to 40.37 mW when the pump power increases from 372 to 659 mW. At a maximum pump power of 659 mW, the average output power is 40.37 mW, and the measured largest output pulse energy was 23.9 nJ, which was limited by the pump power level. The pulse spectrum FWHM evolution with pump power was shown in the Fig. 6(b). The duration of the mode-locked pulse broadens gradually with the pump power increasing from 372 to 602 mW due to the influence of Group Velocity Dispersion (GVD), while the peak of the pulse almost remains constant. The measured results are shown in Fig. 6(c).
Fig. 6. (a) The average output power and the pulse energy versus the pump power, (b) The pulse spectrum FWHM evolution with pump power, (c) The pulse width as a function of pump power.
Table 1 summarizes the reported optical performance of the passively mode-locked Er-doped lasers based on TIs saturable absorbers. As is shown, maximum average output powers of 0.8 and 65 mW and pulse energy of 3.22 to 17.2 nJ have been obtained [30,31,34,41, 43, 44, 52–63]. Through this contrast, we also found out that the 23.9 nJ pulse energy obtained in our work was the largest of all. The results show that Bi2Te3 has the potential for obtaining large pulse energy fiber lasers.
Table 1. Comparison of passively mode-locked Er-doped ring-cavity lasers based on IT SAs
In conclusion, Bi2Te3 film was successfully prepared by CVD method and employed as a saturable absorber within an Er-doped fiber laser for generating passively mode-locked operation. Nonlinear absorption properties of the Bi2Te3 film have been measured, the modulation depth and saturation intensity were 10.8% and about 5.8 MW/cm2. Passive mode-locked operation with a narrowest pulse width of 3.22 ns under a repetition rate of 1.704 MHz was obtained under the pump power of 115 mW. The maximum average output power and pulse energy were as high as 40.37 mW and 23.9 nJ when the pump power was 659 mW, respectively, which were all much higher than in previously reported works. Our result exhibited that Bi2Te3 with excellent nonlinear saturable absorption characteristics will have extensively wide ultrafast photonics and optoelectronic applications.
Funding
National Natural Science Foundation of China (NSFC) (11674199, 11774208, 11804200, 11874244); Natural Science Foundation of Shandong Province (2017GGX20120A, ZR2016AM19, ZR2017BA004, ZR2017BA018); Shandong Province Higher Educational Science and Technology Program (J18KZ011).
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