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Abstract
We experimentally demonstrate that indium selenide (In2Se3), a III−VI group layered chalcogenide compound, can be used as a saturable absorber (SA) for a wavelength-switchable vector-soliton fiber laser. The modulation depth of the In2Se3-based SA (In2Se3-SA), fabricated by incorporating In2Se3 nanosheets with polyvinyl alcohol, is up to 14%. By inserting the In2Se3-SA into fiber laser, solitons switched at wavelengths of ~1558 and ~1530 nm, with the duration of ~1.88 and ~1.76 ps respectively, can be obtained by adjusting the polarization controller and the pump power. Further investigations demonstrate that the achieved solitons are polarization-locked vector conventional solitons, which reveals that the In2Se3 can serves as a polarization-independent SA. The results indicate that the III−VI group chalcogenide compounds, including In2Se3, could be developed as an alternative for ultrafast pulse generations, particularly, vector-soliton pulse.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Based on the rapid rise of graphene, two-dimensional (2D) materials with sheet-liked structures have attracted considerable attention in electronics, optoelectronics and biomedicine [1,2]. Most of them, such as graphene [3,4] topological insulators (TIs) [5–7], transition metal dichalcogenides (TMDs) [8,9], and black phosphorus (BP) [10–12], possess excellent nonlinear optical properties, which have been employed as saturable absorbers (SAs) for the generation of ultrashort pulse. Due to the low absorption coefficient of graphene-based SAs, environmental instability of BP and the limitation of bulk TIs, TMDs have been investigated intensively by many researchers [13,14]. Owing to the layer-dependent bandgap and edge-state defect induced sub-bandgap, SAs built on few-layer TMDs exhibit outstanding performances in ultrafast fiber lasers [15]. Zhang et al. have systematically investigated the nonlinearity of MoS2 nanosheets utilizing balanced-detector measurement technique and open-aperture Z-scan, and demonstrated a MoS2 based passively mode-locked ytterbium-doped fiber laser (YDFL) [16]. Mao et al. have experimentally revealed that MoTe2 and WTe2 possess saturable absorption property and can work as promising SAs for ultrafast erbium-doped fiber lasers (EDFL) [17]. Luo et al. have made a highly nonlinear saturable absorption photonic device by depositing the few-layer MoS2 nanosheets onto the microfiber, and obtained various soliton patterns in a fiber laser with this device [18].
Recently, III−VI group layered chalcogenide compounds (MX or M2X3, where M = Ga or In, and X = Se, Sb, or Te) have drawn strong interest, and they have been discovered to possess some properties in structure and bandgap similar to that of TMDs [19–21]. Indium selenide (In2Se3) is a typical representative of them, and the crystal In2Se3 is composed of vertically stacked Se-In-Se-In-Se quintuple layers [22]. Like the TMDs, the atoms in-plane are held together with strong covalent bonds and the neighboring layers are stacked by the weak Van der Waals interaction, which allows In2Se3 can also be exfoliated into mono- or few-layer nanosheets for developing high-performance optoelectronic devices [23,24]. Moreover, the bandgap of the In2Se3 also exhibits strong layer-dependent property, which is 1.45 eV for thicker nanosheets and 2.8 eV for nanosheets as thin as 3.1 nm [25]. Additionally, the studies of In2Se3 mainly focus on its exciting physical characteristics, such as the topological insulating effect and excellent application prospect on sensors, energy, and catalysis [26,27]. However, few efforts have been made to explore its applications in nonlinear and ultrafast optics field. Therefore, it is of great significance to reveal the nonlinear optical properties of In2Se3, and develop its abilities in the generation of ultrashort pulses, particularly, vector solitons.
In this paper, we demonstrate that In2Se3 can be utilized as a SA for wavelength-switchable vector-soliton fiber laser. This SA is fabricated by incorporating In2Se3 nanosheets with polyvinyl alcohol (PVA), and its modulation depth is up to 14%. By inserting the In2Se3 based SA (In2Se3-SA) into fiber laser, solitons centered at ~1558 and ~1530 nm, with the bandwidths of ~1.5 and ~1.6 nm respectively, can be obtained by tuning the polarization controller (PC) and changing the pump power. The vector properties of the obtained solitons are experimentally investigated, and these solitons are demonstrated to be polarization-locked vector conventional solitons.
2. Characterization of the In2Se3
As shown in Fig. 1, Raman spectrum is applied to reveal the quality and property of the In2Se3 nanosheets, which is investigated by a 532 nm laser at room temperature. Three dominant Raman peaks at 104, 181 and 198 cm−1 can be observed on the spectrum, which are attributed to the A1 (LO + TO), A1 (TO) and A1 (LO) phonon modes in α-In2Se3, respectively [28,29]. According to the intensities of three prominent Raman peaks, the thickness of the In2Se3 nanosheets can be roughly estimated to be around 15–24 nm [29]. The In2Se3 nanosheets used in the experiment is commercially available and their ethanol-water dispersion appears as light brown, as displayed in the inset of Fig. 1(a). Figure 1(b) reveals the linear optical transmittance of the In2Se3 film, which is measured by a spectrometer (Bruker Vertex 80). The film is fabricated by mixing the dispersion of In2Se3 nanosheets with aqueous solution of PVA and then evaporating them on a substrate. First, 0.125 wt% In2Se3 dispersions and 5 wt% aqueous PVA solution are blend at the volume ratio of 3:1 by a magnetic stirrer for 1 hour. Second, drop the mixture on a flat and clean substrate. A thin In2Se3-PVA film is formed after slow evaporation under ambient temperature and pressure for about three days. The film is displayed in the right inset of Fig. 1(b), and the micrograph of its side profile is shown in the left inset of Fig. 1(b). It can be seen that the thickness of the In2Se3-PVA film is about 40 µm. As shown in Fig. 1(b), the transmittance coefficient increases along with the wavelength from 700 to 1200 nm, and stabilizes at around 80% finally. The nonlinear absorption of the In2Se3-PVA film is experimentally measured at different input powers by sandwiching it between two fiber ferrules. A homemade ultrafast fiber laser is employed as the incident light source, whose center wavelength, fundamental repetition rate, pulse width and pulse energy are ~1560 nm, 13.16 MHz, 1.435 ps and ~45 pJ respectively. As illustrated in Fig. 1(c), it can be seen that the In2Se3-PVA film exhibits typical characteristics of saturable absorption. The modulation depth (α0), nonsaturable absorption (αns), and saturation intensity (Isat) of the In2Se3-SA are approximately 14%, 11.6%, and 0.8 MW/cm2, respectively. The difference between the linear transmission at around 1550 nm and the transmission at low peak intensity, as shown in Figs. 1(b) and 1(c), is mainly caused by the existence of the fiber ferrules. Furthermore, we find that the nonlinear absorption properties of the In2Se3-SA are essentially similar, even if the polarization state of the incident light changes, which indicates that the In2Se3 can be developed as a polarization-independent SA.
Fig. 1 (a) Raman spectrum of In2Se3 nanosheets, inset: ethanol-water dispersion of In2Se3 (b) linear transmission of In2Se3-PVA film, inset: In2Se3-PVA films (left) and micrograph of its side profile (right); (c) nonlinear absorption property of the In2Se3-SA.
3. Experimental setup
The schematic diagram of the proposed fiber laser system is shown in Fig. 2. A 980 nm laser diode (LD) is coupled into the laser cavity by a 980/1550 nm wavelength-division-multiplexer (WDM). A 10.8-m-long EDF (Nufern:EDFC-980-HP) with dispersion parameter of −16 ps/nm/km is used as the gain medium. The In2Se3-SA, which is formed by sandwiching an In2Se3-PVA film between two fiber ferrules inside a connector, is inserted into the laser cavity to serve as a mode-locker. A PC is placed between the EDF and WDM to optimize the mode-locking conditions by regulating the polarization state of the ring cavity. The unidirectional operation of the ring cavity is ensured by a polarization-independent isolator (PI-ISO). A 10/90 optical coupler (OC) is utilized to extract optical signals from the laser cavity. All other fibers are standard single-mode fiber (SMF) with the total length of 100 m and dispersion parameter of 17 ps/nm/km. So the total length of the cavity is about 110.8 m, and the net cavity dispersion is estimated as −1.95 ps2. An optical spectrum analyzer (Yokogawa AQ6370B), an autocorrelator, a 6-GHz oscilloscope (R&S RTO2064), a radio-frequency (RF) analyzer, and a 12-GHz photodetector are employed to monitor the laser output simultaneously.
4. Experimental results and analyses
When the pump power is increased to 27 mW, mode-locking operation centered at ~1558 nm can be obtained via carefully tuning of the PC. The low mode-locking threshold is dominated by the low insert loss, small nonsaturable absorption and low saturation intensity of the In2Se3-SA [30,14]. As usual, a multiple-pulse state emerges in the laser cavity. The single pulse operation could be achieved by decreasing the pump power to 8.5 mW, which is the pump hysteresis phenomenon [31]. The typical spectrum of the pulse is displayed in Fig. 3(a), and the 3-dB bandwidth is about 1.5 nm. Obvious Kelly sidebands distribute symmetrically at both sides of the spectrum, which is a typical feature of the soliton fiber lasers with net anomalous dispersion, indicating that the pulses acquired in the laser cavity are conventional solitons [32]. The autocorrelation trace of the soliton pulse is shown in Fig. 3(b). If the sech2 profile is assumed for fitting, the duration of the pulse is ~1.88 ps. The time-bandwidth product (TBP) is ~0.347, which is larger than the transform limit value of 0.315. Therefore, the conventional soliton is slightly chirped. The oscilloscope trace in Fig. 3(c) shows that the pulse intensity is equal and the pulse interval is ~539 ns. Figure 3(d) presents the RF spectrum on a span of 100 Hz with a resolution of 1 Hz. The fundamental repetition rate of this conventional soliton is ~1.853911 MHz, corresponding to the equally spaced pulse interval. The signal-to-noise ratio (SNR) of RF spectrum is > 60 dB, implying a low-amplitude fluctuation and good mode-locking stability. In order to further verify the stability of this operation, a wideband RF spectrum up to 100 MHz is presented in the inset of Fig. 3(d). No spectrum modulation can be observed, which indicates that no Q-switching instabilities exist in the laser cavity.
Fig. 3 Conventional soliton centered at ~1558 nm. (a) Spectrum; (b) autocorrelation trace of the soliton; (c) oscilloscope trace; (d) fundamental RF spectrum, inset: wideband RF spectrum.
With appropriate orientation and pressure setting of the PC, the operating wavelength can be switched from ~1558 to ~1530 nm in the laser cavity when the pump power is increased up to 40 mW. The spectrum centered at ~1530 nm with ~1.6 nm 3-dB bandwidth is exhibited in Fig. 4(a). Based on the measured autocorrelation trace shown in Fig. 4(b), the duration of this soliton pulse is ~1.76 ps. The pulse duration here is limited by the ratio of dispersion and self-phase modulation [33].The TBP is calculated as ~0.36, indicates that this conventional soliton is also slightly chirped. The oscilloscope trace with pulse interval of ~539 ns is presented in the inset of Fig. 4(b). The fundamental repetition rate of this soliton is about 1.854056 MHz, which corresponds to the pulse interval. It can be seen that the solitons of two wavelengths operate at different repetition rates with a separation of ~145 Hz. The chromatic dispersion of the two wavelengths attributes to the slight difference of the repetition rate. According to previous studies, the formation of this switchable mode-locking operation is dominated by the varying gain spectrum of EDF and the changing loss induced by PC [34,35]. It's worth noting that the photon energy around 1550 nm (~0.8 eV) is below the bandgap of In2Se3. Various theories have been used to interpret such sub-bandgap absorption, such as edge-state defect and topological insulating property [36,37].
Fig. 4 Conventional soliton centered at ~1530 nm. (a) Spectrum; (b) autocorrelation trace of this conventional soliton, inset: oscilloscope trace.
In order to verify the vector properties of the solitons, the output signals are passed through a polarization beam splitter. The polarization-resolved spectra of the soliton centered at ~1558 nm are displayed in Fig. 5(a). As shown in Fig. 5(a), extra sets of sidebands emerge on the spectra of two orthogonal polarization components apart from the Kelly sidebands, whose positions vary with the intensity of linear birefringence. While the horizontal component has a spectral peak, a spectral dip is observed on the other component, as revealed in the inset of Fig. 5(a). The appearance of these peak-dip sidebands results from the coherent energy exchange between the two polarization components of the vector solitons [38]. Two polarization-resolved oscilloscope traces with span of 20 µs are illustrated in Fig. 5(b). Both of them have uniform pulse intensity without any modulation, which indicates that the polarization of both components is fixed along the cavity, and these solitons are polarization-locked vector conventional solitons [11]. Similar to the soliton centered at 1560 nm, the spectra of two orthogonal polarization components of the soliton at ~1530 nm also contain Kelly sidebands and extra peak-dip sidebands, as revealed in Fig. 5(c). The oscilloscope traces of two orthogonal polarization components are similar to that shown in Fig. 5(b), and the intensities of the pulses also do not change in each roundtrip time, confirming that the polarization of the vector soliton centered at ~1530 nm is also locked [11]. Polarization selection effect may be introduced into the laser cavity by the polarization-dependent absorption of the SA and would prevent the formation of vector solitons [10]. This further proves that the In2Se3 can be utilized as a polarization-independent SA.
Fig. 5 (a) Polarization-resolved spectra of soliton centered at ~1558 nm; (b) oscilloscope traces; (c) polarization-resolved spectra of soliton centered at ~1530 nm.
Recently, in addition to graphene, TIs, TMDs, and BP, many excellent fiber lasers have been made by another novel 2D material―SnS2 [39–41]. In order to compare the performance of the fiber lasers based on SnS2 and In2Se3, important parameters of three reported SnS2-based fiber lasers and this In2Se3-based fiber laser are shown in Table 1. It can be seen that the In2Se3 can be used as a promising alternative for low threshold and high SNR fiber lasers, which is beneficial from the large modulation depth, low insert loss, small nonsaturable absorption and low saturation intensity of the In2Se3-SA.
Table 1. Comparison between the SnS2 and In2Se3 based fiber lasers
5. Conclusion
In summary, we have experimentally reported the usage of In2Se3 nanosheets as a SA to achieve wavelength-switchable vector-soliton mode-locking in an EDFL. The saturable absorption features of the In2Se3-SA, fabricated by incorporating In2Se3 nanosheets with PVA, have been investigated. The In2Se3 based fiber laser is capable of generating wavelength-switchable solitons, which center at ~1558 and ~1530 nm with the duration of ~1.88 and ~1.76 ps, respectively. Furthermore, the vector properties of the acquired solitons are experimentally investigated, and these solitons are demonstrated to be polarization-locked vector conventional solitons. The results show that the III−VI group chalcogenide compounds, including In2Se3, could be developed as a polarization-independent SA, and it is suitable for the research of vector-soliton fiber laser.
Funding
National Natural Science Foundation of China (NSFC) (61475188, 61635013, 61805277); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB24030600); CAS Light of West China Program (XAB2017A09).
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