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Abstract
In our work, Cr2Si2Te6, a novel two-dimensional (2D)-layered ferromagnetic insulator, was used as a saturable absorber (SA) for demonstrating a pulsed Er-doped fiber laser (EDFL) based on its nonlinear saturable absorption properties. The modulation depth and saturation intensity of the Cr2Si2Te6-based SA are 6.61% and 10.77 MW/cm2, respectively. When the pump power is 60.3 mW, the 13th harmonic mode-locked generation is obtained, the 3 dB optical spectrum bandwidth is 1.392 nm and the central wavelength is 1568.032 nm. When the pump power is higher than 170.3 mW, the EDFL works at the fundamental repetition frequency of 1.23 MHz with a signal-to-noise ratio of 25 dB. The experimental results reveal that Cr2Si2Te6 SA has enormous potential in demonstrating ultrafast fiber lasers and provide references for future research based on Cr2Si2Te6 as ultrafast optical modulators.
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1. Introduction
In the past few years, due to the development of two-dimensional (2D) materials, 2D materials based SAs have been extensively employed for generating picosecond and femtosecond pulses in mode-locked fiber lasers [1–3]. Compared with actively mode-locked fiber lasers, 2D materials-based passively mode-locked fiber lasers exhibit obvious advantages including compact structure, high efficiency, low cost and so on [4–8]. Besides, 2D materials-based passively mode-locked fiber lasers exhibit excellent performance in demonstrating different solitons phenomena and important applications in the fields of photo-chemistry, optical fiber communication and biomedical diagnostics [9–11].
Inspired by the ultrafast photonic applications of graphene [12–14], various kinds of multi-elemental or mono-elemental 2D materials have been used in demonstrating the pulse operation in fiber lasers, such as transition metal dichalcogenides (TMDs) [15–17], topological insulators (TIs) [18–20], black phosphorus (BP) [21–22], transition metal chalcogenide [23–24], MXenes [25–26], antimonene [27–28], and bismuthene [29–30]. Based on 2D materials SAs, different solitons including traditional soliton [31–32], dissipative solitons [33] and so on have been widely reported. Among them, in comparison with the mentioned 2D materials, 2D ferromagnetic semiconductors Cr2Si2Te6 (X = Si and Ge) exhibits special ferromagnetism, semiconducting, optical and electronic characteristics [34–38]. In addition, 2D ferromagnetic semiconductors also have the advantages of low cost, easy preparation, stability under illumination and so on. Recently, ferromagnetic insulators are investigated as SAs in fiber lasers. Ma et al. generated 881 fs mode-locked operation with a CGT-based ultrafast optical modulator [35]. Recently, Guo et al. obtained 550 fs soliton mode-locked operation within an EDFL with a CGT-polyvinyl alcohol (PVA) film type SA [37]. Fu et al. obtained a stable bright-dark soliton pair in an Er-doped fiber laser (EDFL) with a (Cr2Ge2Te6 (CGT)) optical modulator in 2019 [38]. In 2020, based on a CGT-based SA, Zhao et al. successfully achieved dark-bright-bright solitons within an EDFL [39].
However, to our best knowledge, Cr2Si2Te6 (CST), as a member of ferromagnetic semiconductors with a larger magnetic entropy change than CGT, has never been used as SAs in fiber lasers [40]. Cr2Si2Te6, exhibits nearly-2D hexagonal structure [41] and an indirect bandgap value of 0.6 eV, lower than 0.7 eV of Cr2Ge2Te6, which is suitable for generating near-infrared pulsed laser operation [42]. For Cr2Si2Te6, the distance between two layers is 6.8 Å and the layered structure is combined with weak interplanar van der Waals bonds [43]. Its magnetic interaction and magnetic structure have been investigated [41] and it is also used to investigate the modulation of Dirac electrons in epitaxial Bi2Se3 ultrathin films [44]. Similar to Cr2Ge2Te6 and other reported 2D materials, Cr2Si2Te6 also possess excellent optoelectronic characteristics and nonlinear absorption performance [45–46]. Thus, the exploration of the nonlinear optical absorption properties of Cr2Si2Te6 will bring great significance in promoting the investigation of 2D materials-based SAs.
In our work, Cr2Si2Te6-based SA, with a modulation depth of 6.61% and a saturation intensity of 10.77 MW/cm2, was used within an Er-doped fiber laser for obtaining pulsed operations. The EDFL operated at high harmonics when the pump power was between 40.7 and 170.3 mW with the calculated pulse width of 1.85 ps. When the pump power was higher than 170.3 mW, the EDFL works at the fundamental repetition frequency of 1.23 MHz with a signal-to-noise ratio (SNR) of 25 dB. Experimental results proved that Cr2Si2Te6 SA had enormous potential for obtaining pulsed operation in fiber lasers.
2. Material preparation and characterization
In our work, the Cr2Si2Te6-polyvinyl alcohol (PVA) film-type optical modulator was fabricated by the methods of liquid-phase exfoliation and spin coating technology. Firstly, 100 mg Cr2Si2Te6 powder is added into 100 mL alcohol (30%) for 24 h, then Cr2Si2Te6 dispersion solution was obtained by disposing the Cr2Si2Te6-alcohol within an ultrasonic cleaner for 5 h. Then, the mixture was placed in an ultrasonic cleaner at the speed of 1500 rpm for 6 h to remove the precipitation. After that, the Cr2Si2Te6 dispersion was mixed with 5 wt.% PVA solution at a volume ration of 1:1. The mixture was further ultrasonically-mixed for 3 h for preparing a uniform Cr2Si2Te6-PVA dispersion. After that, 90 µL Cr2Si2Te6-PVA solution was spin coated on a sapphire substrate and set into an oven for 24 h at 30 °C. Finally, 1×1 mm2 Cr2Si2Te6-PVA film was cut off and put on the end face of the fiber for proposing a home-made ultra-fast optical modulator is fabricated successfully.
The characteristics of the Cr2Si2Te6 nanosheet are characterized. Figure 1(a) shows the surface characteristics of Cr2Si2Te6 nanosheets analyzed by a scanning electron microscope (SEM). Under a resolution of 2 µm, obvious layered structure is provided. The corresponding spectroscopy of energy dispersive spectrometer (EDS) is depicted in Fig. 1(b), obvious peaks of Cr, Si, Te are described. The stoichiometric ratio of Cr (20.66%), Si (19.85%) and Te (59.49%) is estimated to be 1:1:3. The crystal structure of Cr2Si2Te6 nanosheets is analyzed by X-ray diffraction (XRD) and the diffraction XRD spectrum is depicted in Fig. 1(c). As shown in Fig. 1(c), Typical peaks at 2θ = 12.94, 25.96 and 29.52° corresponding to (003), (006) and (113) planes in Cr2Si2Te6 are obtained, respectively. The high diffraction peak at the (006) plane indicates that Cr2Si2Te6 nanosheets with well-layered structures are successfully obtained. And the (003) plane indicates that the layered Cr2Si2Te6 nanosheets exhibit excellent crystallinity. Figure 1(d) shows the Raman spectrum. There are three obvious peaks locating at 80.54, 120.91, and 485.03 cm−1.
Fig. 1. (a) SEM image of Cr2Si2Te6 nanosheets, (b) EDS spectroscopy of Cr2Si2Te6 nanosheets, (c) XRD spectrum, (d) Raman spectrum of Cr2Si2Te6 nanosheets,
In addition, the nonlinear optical absorption characteristics of the Cr2Si2Te6-PVA film is tested based on the power-dependent transmission technique and a femtosecond laser source. The pulse width, central wavelength, and repetition rate are 460 fs, 1556.7 nm, and 20.18 MHz, respectively. Figure 2 shows the relationship between the input power and the transmission through the Cr2Si2Te6-PVA film. The experiment date are fitted by the formula [15]:
where T(I) is the transmission rate, Tns is the non-saturable absorbance, Δ is the modulation depth, I is the input intensity, and Isat is the saturation intensity. Based on the formula, the saturation intensty and modulation depth can be achieved. In our experiment, the non-saturable absorbance, saturation intensity and modulation depth are 58.33%, 10.77 MW/cm2, 6.61%, respectively.3. Experimental setup and results
The experiment schematic of the mode-locked EDFL based on the Cr2Si2Te6-PVA SA is shown in Fig. 3. The total length of the ring cavity is about 167.3 m including a 10 m long EDF (MP980) as the gain medium and a 157.3 m long SMF. Because the formation of different mode-locked soliton states depends on the gain, loss and net dispersion in the laser resonator, in our experiment, we realized the control of the gain, loss and net dispersion in the laser resonator by optimizing the length of gain fiber and single-mode fiber. The dispersion parameters of SMF and EDF are 17 and −18 ps/(nm·km) at 1550 nm, respectively. The net dispersion of the cavity is calculated to be −3.18 ps2. Through a 980/1550 nm wavelength division multiplexer (WDM), the EDF is pumped by a 976 nm laser diode (LD) with a maximum output power of 600 mW. The Cr2Si2Te6-PVA film is set between the two fiber connectors using as the SA. A polarization-independent isolator (PI-ISO) is inserted behind the SA to ensure the unidirectional operation of the pulse in the ring cavity. The polarization state is adjusted by a polarization controller (PC). Besides, an output coupler with an output radio of 80:20 is used to output the laser through its 20% port. The output characteristics of the mode-locked operation are monitored by an optical spectrum analyzer (Yokogawa AQ6370B), a digital oscilloscope (Wavesurfer 3054z), a radio frequency (RF) spectrum analyzer (Rohde & Schwarz FPC1000) and an optical power meter.
In our experiment, firstly, output characteristics of the ring-cavity fiber laser were investigated without inserting the Cr2Si2Te6-PVA film into the laser cavity, no pulsed operations were observed by adjusting the pump power and the polarization state of the PC, indicating that no self-mode locked or Q-switched phenomena were occurred. Then, the Cr2Si2Te6-PVA film was inserted into the laser cavity, the insert loss of the Cr2Si2Te6-PVA film based saturable absorber was measured to be about 1.68 dB. By adjusting the polarization state of the PC, mode-locked operation was obtained when the pump power was increased to be 40.7 mW. As is mentioned, the formation of soliton depends on the common effect of nonlinear effects, dispersion and so on, and the dispersion is mainly controlled by controlling the degree of polarization controller. In our experiment, the degree of polarization controller is adjusted from 0 to 180. Low mode-locked threshold was due to the low insert loss and low saturation intensity of the home-made Cr2Si2Te6-PVA film.
Figure 4 shows pulse trains with the repetition frequency of 15.93 MHz and the pulse separation is 62.78 ns, corresponding to the 13th harmonic. The optical spectrum of the 13th harmonic is shown in Fig. 5(a), distinct Kelly sidebands are caused by the periodic equilibria of the dispersion and nonlinearity effect, which indicates that the pulse has a typical traditional soliton-like shape. The full width at half maximum (FWHM) is 1.392 nm, the central wavelength is 1568.032 nm. According to the time bandwidth product (TBP), the limiting pules duration can be calculated as 1.85 ps. However, as is shown, the optical spectrum is tilted and the first-order Kelly sideband is strong which is caused by higher-order nonlinear optical effects. Continuously increasing the pump power, higher harmonics are obtained. The changes of optical spectrum with the increase of the pump power are shown in Fig. 5(b), the central wavelength is 1568.032 nm, which indicates that stable mode-locked operation is obtained in our experiment.
When the pump power is 170.3 mW, the Er-doped fiber laser operates at the fundamental repetition frequency of 1.23 MHz with a pulse separation of 813.77 ns. The pulse train is shown in Fig. 6(a). Besides, the RF spectrum with a signal-to-noise ratio of 25 dB is shown in Fig. 6(b), which indicates that and the EDFL operates at a stable state. When the pump power is higher than 300 mW, no stable mode-locked operation is obtained. However, when the pump power is adjusted to 40.7 mW, stable mode-locked operation is obtained, indicating that the SA can be used as SA under high pump power. Figure 6(c) shows the relationship between the the average output power and the pump power, the average output power increases almost linearly as the pump power increases from 40.7 mW to 300 mW.
Fig. 6. (a) Pulse train at fundamental repetition rate; (b) fundamental RF spectrum; (c) average output power.
The experiment results reveal that the Cr2Si2Te6-PVA film is an excellent SA for demonstrating mode-locked traditional soliton EDFLs operating at 13th harmonic and fundamental repetition frequency. This is the first demonstration focusing the application of nonlinear optical absorption properties of Cr2Si2Te6.
4. Conclusions
In summary, we demonstrate traditional-soliton mode-locked operations based on a novel home-made Cr2Si2Te6-PVA film as SA within an EDFL. The central wavelength is 1568.032 nm with a 3 dB optical spectrum bandwidth of 1.392 nm. 13th harmonic mode-locked operation is obtained when the pump power arranged between 40.7 and 170.3 mW. Additionally, when the pump power is higher than 170.3 mW, the EDFL works at the fundamental repetition frequency of 1.23 MHz. The experimental results reveal that Cr2Si2Te6 as a new member of the ferromagnetic insulators has enormous potential to be an excellent ultrafast optical modulator in fiber lasers.
Funding
National Natural Science Foundation of China (11904213, 61971271); Natural Science Foundation of Shandong Province (ZR2018QF006).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request.
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