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Abstract
Various two-dimensional (2D) materials show unique optical properties and excellent performance in acting as saturable absorber (SA) for demonstrating all-fiber ultra-fast lasers. Tellurene, as a new-fashioned few-layer 2D monoelemental material, was designed as an excellent saturable absorber to achieve Q-switched and mode-locked operations within erbium-doped fiber (EDF) lasers in our experiment. High-quality tellurene-based SA with a modulation depth of 0.97% was obtained by blending few-layer tellurene nanosheet solution prepared by liquid phase exfoliation method and the polyvinyl alcohol (PVA) solution. Inserting the SA into the EDF laser cavity by sandwiching the tellurene-PVA film between two fiber ferrules, either the passively Q-switched or the passively mode-locked operations can be obtained. The repetition rate varies from 15.92 to 47.61 kHz, and the pulse duration decreases from 8.915 to 5.196 µs in the passively Q-switched operation. To the best of our knowledge, this is the first demonstration focusing on the modulation application of tellurene in designing Q-switched pulsed laser operations. Additionally, mode-locked operations were also achieved by adjusting the polarization state. The obtained results fully indicate that tellurene can be developed as an efficient SA for pulsed fiber lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed lasers have been widely used in various fields, such as industrial manufacture, biomedical science [1,2], optical communication [3] and microimaging [4,5]. Thanks to the advantages of simple setup, low cost and mechanical stability, all fiber lasers operated at either passively Q-switched or mode-locked operations based on saturable absorber (SA) have attracted considerable investigations. The SAs, including semiconductor saturable absorber mirrors (SESAMs) [6,7], carbon nano tube (CNT) [8–10] and 2D materials [11–24], play a crucial role in the generations of pulsed lasers. Among them, 2D material based SAs perform excellent properties, such as low cost, ultrafast response time and easy preparation [25,26]. Intensive research was devoted to graphene attributing to its prominent mechanical, electrical and optical properties in comparison with traditional materials. Various novel and distinctive optoelectronic devices with super performance based on graphene were designed and reported [27–34]. Graphene based SA was firstly reported in 2009 [27–29], and it was regarded as the milestone for further investigations of other 2D materials and 2D material-based optoelectronic devices. Various 2D materials have been successfully introduced to serve as SAs inspired by the applications of graphene, including topological insulators (TIs) [14,35–38], transition metal dichalcogenides (TMDs) [39–47], ferromagnetic insulators [48–50], Mxenes [51–54], black phosphorus (BP) [55–58], graphdiyne [59], phosphorene [60–63], antimonene [64,65], bismuthene [66–68], silicene [69,70], and tellurene [71,72].
Recently, layered 2D monoelemental materials (Xenes) have received tremendous attention benefit from its excellent properties, such as nonlinear absorption property and thickness-dependent bandgap value, and thus, Xenes have been widely developed as SAs [60–72]. In 2015, the nonlinear absorption properties of phosphorene were investigated by Zhang et al. [60,61]. Afterwards, the pulsed lasers were widely developed ranging from visible to mid-infrared wavelength employing phosphorene as SAs [60–63]. However, its further applications in the ultrafast photonics were restricted by its air instability property. By employing the antimonene-based SA as a mode locker, traditional soliton (TS) operation with the pulse width of 552 fs under a pulse repetition rate of 10.27 MHz was successfully achieved. Meanwhile, passively Q-switched operation with a pulse duration of 1.31 µs was also reported based on antimonene-based SA in their experiment [64]. Recently, Liu et al. investigated the saturable absorption properties of antimonene-based SA using the Z-scan method, and demonstrated the single- and dual-wavelength mode-locked fiber laser pulses based this kind of SA [65]. In 2017, TS mode-locked operation with a pulse width of 652 fs under a pulse repetition rate of 8.83 MHz was demonstrated by utilizing bismuthene as SA [66]. Guo et al. achieved the stable mode-locked operation with the shortest pulse width of about 193 fs centered at 1561 nm based on bismuthene as SA [67]. All the investigations indicated that bismuthene-based SA possessed prominent nonlinear saturable absorption properties and outstanding potential in the generation of femtosecond-level pulsed lasers. In 2019, silicene-based SAs were also prepared to obtain passively Q-switched all-fiber or solid-state laser operations successfully [69,70]. Experimental results indicated that silicene-based SAs showed favorable capacity in generating pulsed lasers. However, there is no report on the mode-locked operation by silicene-based SA. In 2019, tellurene was successfully prepared based on liquid-phase exfoliation (LPE) method, and proven to exhibit excellent nonlinear saturable absorption properties. Based on tellurene as SAs, passively mode-locked EDF and Yb-doped fiber lasers with pulse durations of 829 fs and 456.6 ps, respectively, were successfully demonstrated [71]. Recently, the large-energy mode-locked operations (dissipative soliton (DS) and noise-like pulses (NLP)) were obtained based on tellurene as SA. For the DS operation, the pulse duration, maximum average output power, and the largest pulse energy are 5.87 ps, 23.61 mW, and 1.94 nJ, respectively [72]. However, up to now, the tellurene-based SA for obtaining Q-switched operation has not been reported. In brief, Xenes have played a significant role in the development of novel photonic devices. However, the investigations for the nonlinear absorption properties and applications of Xenes-based SAs are still of great significance to further develop ultrafast photonic devices.
In this work, we demonstrated that the tellurene-based SA (Te-SA) can be used for the fabrication of Q-switched and mode-locked fiber lasers. This SA was obtained by sandwiching the tellurene-PVA film between two fiber ferrules, and the tellurene-PVA film was made by blending the tellurene nanosheets solution and PVA solution. The nonlinear optical absorption property of Te-SA was investigated. It is found that Te-SA shows obvious saturable absorption property, whose modulation depth and nonsaturable absorption are 0.97% and 42.3%, respectively. By incorporating the Te-SA in the laser ring cavity, pulse generations at either Q-switched or mode-locked operations can be achieved in the same cavity. The Q-switched and mode-locked EDF lasers operating at 1550 nm further demonstrate the potential of tellurene serving as SA for pulse generation in fiber laser.
2. Fabrication and characterization of Te-SA
2.1 Fabrication of Te-SA
The preparation of Te-SA can be decomposed into two main steps: the fabrications of few-layer tellurene nanosheets and Te-SA. The procedures for the fabrication of few-layer tellurene nanosheets are as follows. Firstly, the few-layer tellurene nanosheets is fabricated by the LPE method. The tellurium powder, which is stripped from bulk tellurium, is soaked for about 48 h in ethanol. Then, the tellurene-ethanol solution is dealt with an ultrasonic cleaner for 8 h to improve the dispersal of tellurene nanosheets. Afterwards, the tellurene-ethanol solution is placed in a centrifuge to centrifuge the solution for 30 min at a rate of 2000 rpm, which is an indispensable step to remove the precipitation. Up to now, the high quality few-layer tellurene nanosheets are prepared successfully. Based on the prepared tellurene nanosheets, the process for the fabrication of Te-SA is as follows. The tellurene nanosheets solution is mixed with 5 wt% polyvinyl alcohol (PVA) solution at the volume ratio of 1:1. The mixed solution is ultrasoniced for 6 h in an ultrasonic cleaner to obtain uniform tellurene-PVA solution. The mixed tellurene-PVA solution is dropped on a flat and clean glass substrate. After evaporation under room temperature for about 72 h, a thin tellurene-PVA film is formed. Finally, the Te-SA is prepared successfully.
2.2 Characterization of tellurene
To investigate the quality and properties of the prepared tellurene nanosheets, scan electron microscopy (SEM, Sigma 500, Zeiss), energy-dispersive X-ray spectroscopy (EDS, SU8000, Hitachi), Raman spectrometry (Horiba HR Evolution), X-ray diffraction (XRD, D8 Advance, Bruker AXS), and atomic force microscopy (AFM, Bruker Multimode 8) are employed to test the characterization of tellurene nanosheets, and the results are shown in Fig. 1. Figure 1(a) shows the SEM image of the bulk tellurium. It is found that the bulk tellurium possesses obvious layered structure, which indicates that the tellurene nanosheets can be obtained via simple ways, such as mechanically exfoliation, and LPE. The Raman spectrum, as shown in Fig. 1(b), is investigated by a 532 nm laser at room temperature for both bulk tellurium and tellurium powder. The three Raman peaks, corresponding to E1, A1 and E2, are obviously observed, which is well consistent with the reported standard Raman peaks [73], demonstrating the purity of prepared tellurene. The XRD measurement of tellurium powder, as shown in Fig. 1(c), exhibits the hexagonal crystalline property. The diffraction peaks are consistent with the standard pattern of Te [JPCDS No. 36-1452]. The EDS and SEM results are shown in Figs. 1(d) and 1(e). The EDS result indicates that Oxygen, Aluminum, Platinum and Tellurium existed in the measured sample. During the measurement, the nanosheets solution was dropped on a piece of aluminum foil, and a layer of platinum was coated on the surface for electrical conductivity after drying. Therefore, it further confirms the high purity of the nanosheets. The AFM image is shown in Fig. 1(f). It depicts that the thickness of the tellurene nanosheets is about 5 ∼ 6 nm, corresponding to 12 ∼ 14 layers, as shown in the inset. The tellurene nanosheets show uniform thickness and guarantee the stable and excellent performance of the Te-SA.
Fig. 1. (a) SEM image of bulk tellurium; (b) Raman spectra of stripped tellurium powder and bulk tellurium; (c) Theoretical and measured XRD results of tellurium powder; (d) EDS result of prepared tellurene nanosheets; (e) The SEM image of prepared tellurene nanosheets; (f) AFM image of prepared tellurene nanosheets (Inset: thickness and profiles of prepared tellurene nanosheets).
The nonlinear saturable absorption property of the Te-SA is measured by employing the power-dependent measurement system, as shown in Fig. 2(a). The central wavelength, and repetition rate of the incident laser, generated from a homemade fiber laser, are 1564.8 nm, and 12.5 MHz, respectively. The incident laser is split into two parts by an output coupler (OC). One passes through the Te-SA, and the other is directly measured as the reference. An optical attenuator is used to continuously adjust the input power. Figure 2(b) shows the relationship between the input power and the transmission through Te-SA. The curve is fitted by the following formula [74],
Fig. 2. (a) Schematic of power-dependent saturable absorption measurement system; (b) Nonlinear absorption property of Te-SA.
3. Fiber laser setup
Incorporating the fabricated Te-SA, an all-fiber laser system is designed and constructed, and its schematic diagram is shown in Fig. 3. The pump source of laser diode (LD) centered at 976 nm is coupled into the laser cavity by a 980/1550 nm wavelength division coupler (WDM). The laser gain medium is a 9-m long EDF (OFS MP980). A polarization-independent isolator (PI-ISO) is used to ensure the unidirectional propagation of the light in the ring cavity. The polarization state of the cavity is adjusted by a polarization controller (PC). The testing signal is extracted from the 10% port of a 10/90 OC. The Te-SA is inserted into the ring cavity by sandwiching it between two fiber ferrules. The single mode fiber (SMF) and all the pigtail fiber are all SMF-28, and the total length of the ring cavity is about 40.8 m. The signal extracting from the cavity is monitored by an optical spectrum analyzer (Yokogawa AQ6370B), a digital oscilloscope (Gwinstek MDO-2202EG), a radio frequency spectrum analyzer (RF) (Rohde & Schwarz FPC1000), and a power meter, corresponding to the parameters of spectrum, pulse train, repetition rate and average output power, respectively.
Fig. 3. Schematic diagram of proposed fiber laser setup. LD: laser diode; WDM: wavelength division multiplexer; EDF: erbium-doped fiber; OC: optical coupler; PI-ISO: polarization-independent isolator; PC: polarization controller; SMF: single-mode fiber; SA: saturable absorber.
4. Experimental results and discussions
4.1 Q-switched operation
The proposed EDF laser performs continuous-wave (CW) operation at the pump power of 10 mW. As increasing the pump power, stable Q-switched laser pulse train appears at the pump power of 46 mW. When continuously increasing the pump power, the repetition rate increases accordingly, which is the typical characteristic of a passively Q-switched operation, indicating that the EDF laser works in the stable Q-switched state. Figure 4 summarizes typical characteristics of the passively Q-switched operation at the pump power of 46 mW. Figure 4(a) shows a pulse interval of 62.8 µs, corresponding to a repetition rate of 15.92 kHz. The inset shows the Q-switching pulse train. No obvious peak intensity modulation is observed over a wide range of 2000 µs, demonstrating the high stability of the Q-switched operation. Figure 4(b) depicts the profile of single pulse with a full width at half maximum (FWHM) of 8.915 µs. From the optical spectrum in Fig. 4(c), it can be found that the 3-dB spectral bandwidth is about 0.2 nm centered at 1563.7 nm. For testing the stability of the Q-switched fiber laser, the RF spectrum was implemented, and shown in Fig. 4(d). The fundamental repetition rate is 15.92 kHz with a signal to noise ratio (SNR) of ∼ 44 dB.
Fig. 4. Characteristics of Q-switched operation at pump power of 46 mW. (a) The pulse interval of 62.8 µs between adjacent pulses (Inset: the pulse train in a range of 2000 µs); (b) The pulse with FWHM of 8.915µs; (c) The spectrum with 3-dB spectral bandwidth of 0.2 nm centered at 1563.7 nm; (d) The RF spectrum with repetition rate of 15.92 kHz and the SNR greater than 44 dB.
The evolution of the Q-switched pulse train with the pump power is demonstrated in Fig. 5. The repetition rates are 15.92, 22.99, 34, and 47.61 kHz, respectively, corresponding to the pulse interval of 62.8, 43.5, 29.41, and 21 µs, at the pump power of 46, 96, 146, and 195 mW. The evolution trend, i.e. the repetition rate increasing as the pump power increasing, represents the feature of the Q-switched operation. What’s more, the intensities keep considerably uniform at each pulse train during the process of increasing the pump power, which reasonably draws the conclusion that the Te-SA EDF laser operates at a highly stable Q-switching state.
Fig. 5. The evolution of Q-switched pulse train with the pump power. (a) 46 mW; (b) 96 mW; (c) 146 mW; (d) 195 mW.
Figure 6 shows the evolution of the Q-switched related parameters with the increment of the pump power. The repetition rate, pulse duration, and average output power are all consistent with the characteristics of Q-switched operation. The repetition rate and the average output power almost linearly increase as the pump power increasing. The pulse duration rapidly decreases from 8.915 to 5.196 µs, when the repetition rate varies from 15.92 to 47.61 kHz with the increment of the pump power.
Fig. 6. The evolution of the repetition rate, pulse duration, and average output power with the increment of the pump power.
Table 1 summarizes the performance of different Xene-SA Q-switched lasers, including BP, antimonene, silicene and this work. It is found that adjustment range of the repetition rates for our laser is comparative to that of silicene-SA laser, and better than that of BP and antimonene. However, the output power and conversion efficiency are obviously higher than other reported Xene-SA Q-switched lasers.
Table 1. Comparison of different Xene-SA Q-switched lasers.
4.2 Mode-locked operation
With the same ring cavity and Te-SA as shown in Fig. 3, passively mode-locked operation is also achieved by gradually adjusting the PC at the pump power of 96 mW. Figure 7 summarizes the typical characteristics of the passively mode-locked operation. Figure 7(a) shows the pulse train in 2 µs. The pulse train shows a pulse interval of 198.5 ns, corresponding to the length of the ring cavity, which proves the laser working at the mode-locked operation. The pulse train over 10 µs is shown in Fig. 7(b). It is found that there is barely intensity modulation over a long span, which indicates the high stability of the passively mode-locked operation. The corresponding optical spectrum is shown in Fig. 7(c). The 3-dB bandwidth is about 0.12 nm centered at 1565.58 nm. The pulse duration is estimated to about 21.45 ps according to the theoretical value of time-bandwidth product (TBP). It should be pointed that two splitting peaks are observed in the spectrum, which may be aroused by the self-phase modulation (SPM) effect. The SPM effect can broaden the spectrum and in turn result in multiple peak structure [75,76]. The RF spectrum over 0.08 MHz is shown in Fig. 7(d), and the fundamental repetition rate is 5.0378 MHz with a SNR up to 42.3 dB, which is agreement with the pulse interval. To further prove the high stability of the mode-locked state of the laser, the inset gives the RF spectrum within a bandwidth of 100 MHz.
Fig. 7. Characteristics of the passively mode-locked operation; (a) The pulse interval of 198.5 ns between adjacent pulses; (b) The pulse train in a range of 10 µs; (c) The spectrum with 3 dB spectral bandwidth of 0.12 nm centered at 1565.58 nm; (d) The RF spectrum with repetition rate of 5.0378 MHz and the SNR greater than 42.3 dB (Inset: the RF spectrum with a bandwidth of 100 MHz).
Figure 8 depicts the relationship between the pump power and the average output power. Under the mode-locked operation, the average output power of the laser increases linearly with the increment of pump power over the range 96 ∼ 366 mW. And the maximum average output power is 17.44 mW.
5. Conclusions
In conclusion, we have experimentally demonstrated that tellurene nanosheets can be used as an excellent saturable absorber for the generation of ultrafast pulses. Te-SA can be prepared by the LPE method, which showed uniform thickness and high quality. The nonlinear absorption properties of the Te-SA were investigated by the power-dependent method. The Te-SA showed obvious saturable absorption property, and the modulation depth is 0.97%. The EDF laser based on Te-SA was designed and constructed, which can operate in either Q-switching or mode-locking state. In the Q-switched operation, the repetition rate varied from 15.92 to 47.61 kHz, the pulse duration varied from 8.915 to 5.196 µs, with the increment of the pump power. The study further demonstrated that tellurene is a potential optoelectronics material in the optoelectronics devices.
Funding
National Natural Science Foundation of China (11704227, 11904213).
Disclosures
The authors declare no conflicts of interest.
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