As a new member of transition metal dichalcogenides (TMDs), rhenium disulfide (ReS2), with a nearly unchanged direct bandgap from bulk to monolayer form, is attractive in physics and material fields. By using the optically driving deposition method, the ReS2 saturable absorber (SA) has been fabricated with a modulation depth and saturation fluence of 6.9% and 27.5 μJ/cm2, respectively. Based on the ReS2-SA, a multi-wavelength bright-dark pulse pair from a mode-locked fiber laser has been observed experimentally for the first time, to the best of our knowledge. The saturable absorbing ability of the ReS2 is attributed to the formation of the bright pulses and the dark pulses. The cross-phase modulation (XPM) caused by different wavebands of bright pulse and dark pulse support the coexisting of the bright-dark pulse pair.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Soliton can be generated from many physical systems. In the field of optics, solitons formed in the single mode fiber (SMF) are attractive for its important applications in optical communication, optical continuum generation and optical processing system . To date, various pulse types have been experimentally discovered from passively mode-locked fiber lasers, such as multi-wavelength pulses, femtosecond pulses, high-order soliton and dissipative soliton. Contrary to the bright pulses above, dark pulse has an intensity dip in continuous-wave (CW) background and possesses some advantages, such as more stability in the presence of noise and more slow spreading in the presence of fiber loss . According to the soliton interaction, there are some pulse pairs in optical fiber system, such as bright-bright, dark-dark, and bright-dark pulse pair. The pulse pairs have been investigated theoretically through nonlinear Schrodinger equation (NLSE) . The bright and dark pulse pairs have been realized in fiber laser through mode-locking technique [4, 5]. Based on the previous reports, the XPM effect or polarization effect may attribute to the generation of bright-dark pulse pair [6, 7].
Recently, a newly emerging member of TMDs, rhenium disulfide (ReS2) has shown an intrinsic feature of direct bandgap semiconductor, which is independent on the crystallographic structure. The value of bandgap is in the range of 1.35 - 1.43 eV from bulk to monolayer form, which shows an almost unchanged electronic energy structure [8, 9]. Very recently, the first ReS2 mode-locked fiber laser in the infrared band has been reported . Shortly, D. Mao et al. and F. F. Lu et al. studied the Q-switching and harmonic mode-locking characteristics of ReS2 in fiber lasers, respectively [11, 12]. These works exhibit excellent properties of ReS2 for ultrafast fiber lasers. However, only bright pulses have been obtained. Therefore, it is very interesting to explore the possibility of generating other soliton types by using ReS2 in fiber lasers.
In this paper, a multi-wavelength bright-dark soliton pair in an erbium doped fiber (EDF) laser has been successfully generated based on ReS2. The ReS2-SA was fabricated by using liquid exfoliation and optically driving deposition methods, with modulation depth and saturation fluence of 6.9% and 27.5 μJ/cm2, respectively. The generation of dark pulses is probably in virtue of nonlinear effect of ReS2. The bright pulse and the dark pulse at different wavelengths have been obtained by adjusting an external cavity polarization controller (PC), which may cause the XPM effect supporting the co-exiting of the bright-dark pulse pair. Our work provides the first result that bright-dark pulses could co-exit simultaneously in the passively mode-locked fiber laser with ReS2.
2. Preparation and characterization of ReS2-SA
The layered nanomaterials are easily delaminate due to the strong in-plane chemical bonds but weak out-of-plane van der Waals bonds. In this work, the ReS2 nanomaterial was prepared using the liquid exfoliation method for its low cost and simple operation as described in . Firstly, the ReS2 powder was dissolved in the solvent of a mixture of alcohol and deionized water with a volume ratio of 7:3. A high-power-ultrasonic cleaner was used for sufficiently dissolving ReS2 powder. After the sonication for 12 h, the ReS2 solution was centrifuged at 2000 rpm with 20 min for separation of the ReS2 nanosheets with large agglomeration. By decanting the upper supernatant, the homogeneously dispersed ReS2 nanosheets solution was fabricated. For testing the characterization of ReS2 nanosheets, the as-prepared dispersion was dropped onto a sapphire substrate to form a ReS2 sample. The Raman spectrum was characterized with an excitation source at the wavelength of 532 nm. The results are shown in Fig. 1(a) with six peaks locating at 138.1, 143.5, 450.9, 160.8, 211.8, and 234.6 cm−1, which shows a negligible shift from previously report . Because the Raman spectrum is insensitivity to the layer number of ReS2, the Raman shift may be caused by the different polarization of probe laser [11, 14]. The morphology of ReS2 observed by atomic force microscope (AFM) is shown in Fig. 1(b). From Fig. 1(d), it can be seen the thickness was about 5 nm, corresponding to 7 layer ReS2 . The linear transmittance of the ReS2-sample was also investigated by a spectrophotometer (U-3500) in comparison with the bare sapphire. The ReS2 sample transmittance shows an ultra-broad absorption band beyond its bandgap.
Besides, the nonlinear optical property of ReS2 was also investigated. The measurement setup is shown in Fig. 2(a). The probe laser with an average power of 20 mW was a home-made passively mode-locked fiber laser. The repetition rate and the pulse width were 41.5 MHz and 1 ps, respectively. The measured results are shown in Fig. 2(b). According to the fitting formula , the modulation depth and saturation fluence were estimated to be 6.9% and 27.5 μJ/cm2, respectively, confirming the possibility of the ReS2 as a SA for ultrashort pulse generation.
3. Experimental setup
For being used in fiber laser the ReS2-SA was fabricated by optically driven deposition method. In details, light from a 974 nm laser diode (LD) with an output power of 50 mW was injected into the fiber, and the fiber ferrule was dipped into the ReS2 dispersion for 15 min. After drying at the room temperature for 24 h, it was connected with a clean ferrule by an optical adaptor to form ReS2-SA, which was incorporated into the fiber laser cavity.
The experimental schematic setup of the mode-locked fiber laser is presented in Fig. 3. A simple ring cavity with all fiber structure was configured. A fiber-pigtailed 976 nm LD was used as the pump source with a maximum pump power of 500 mW. A piece of 95-cm EDF was pumped via a wavelength division multiplexer (WDM) with dispersion parameter of −12 ps/nm/km. A PC device and a polarization-insensitive isolator (ISO) were employed to match the polarization state and force the unidirectional light propagation. An output coupler (OC) with a coupling rate of 20% was used for outputting laser signal. The rest of fibers in the cavity were all standard SMF with dispersion parameter of 18 ps/nm/km. The net cavity dispersion was −0.317 ps2 for the total cavity length of 15.4 m. The laser performance was detected by a 1 GHz digital oscilloscope (Tektronix DPO 7104), a 3 GHz RF spectrum analyzer (Agilent N900A) coupled with a 1 GHz photodetector and an optical spectrum analyzer (Yokogawa AQ6370C).
4. Results and discussions
The dependences of output powers on the pump powers for the fiber lasers with and without ReS2 SA were firstly studied, as shown in Fig. 4(a). When ReS2 SA was inserted, the threshold for oscillation and slope efficiency were found to be 45.2 mW and 6.04%, respectively. The fiber laser ran into mode-locking regime at the pump power of 120 mW. By carefully adjusting the PC, stable bright-dark pulse pair could be observed. However, once the pump power exceeded 270 mW, the pulse trains collapsed suddenly, and the stable pulses still could not be reconstructed by decreasing the pump power, which was attributed to the thermally induced damage to the ReS2-SA under high power intensity. According to 20%-output-couple and 8.2-μm-core-diameter, the damage threshold of ReS2-SA in our experiment can be calculated to be 7.7 × 103 MW/cm2.
At the pump power of 270 mW, as described in Fig. 4(b), a bright pulse together with a dark pulse was observed simultaneously, which was called bright-dark pulse pair . The interval of pulse train was 74.6 ns, exactly corresponding to the round-trip time of the cavity. The single pulse profile depicted in the inset of Fig. 4(b) clearly shows the asymmetric outline of the bright-dark pulse pair. The pulse shapes and amplitudes of the bright pulse and dark pulse were all different, which made them support each other through the XPM coupling . The output optical spectrum with a multi-wavelength emission is shown in Fig. 4(c). Three spectral peaks were located at 1573.6, 1591.1 and 1592.6 nm, respectively. The spectral interval of 17.5 nm was much larger than those based on the other nanomaterials, such as black phosphorus (BP) , topological insulator (TI)  and the other TMD (MoS2, WS2) [18, 19]. Besides, the spectrum of CW state (without ReS2-SA) showed only laser at 1594.1 and 1595.5 nm oscillated, which indicated the 17.5-nm large wavelength interval caused by ReS2-SA, not the filtering effect. The results proved the outstanding performance of ReS2 as SA for generating multi-wavelength pulsed laser. Figure 4(d) shows the RF spectrum with a resolution bandwidth (RBW) of 3.6 kHz at the pump power of 270 mW. The pulse repetition frequency was 13.39 MHz and the single-to-noise ratio (S/N) exceeded 55 dB. The results indicated good stability of the mode-locked fiber laser with bright-dark pulse pair. To further investigate the multiwavelength operation, an ultrafine RF spectrum with a RBW of 22 Hz and a span range of 7 kHz was also recorded as shown in the inset of Fig. 4(d). The RF peaks were separated by 744 Hz and 80 Hz, respectively, for different wavelengths. In theory, the relationship between the RF separation interval Δf and wavelength difference dλ is described as follows :7]. The phenomenon may be caused by inherent feature of bright-dark pulse pair in fiber laser.
For further investigation, an external cavity PC and a tunable filter were used to survey the change of the pulse pair. First, the filter has been tuned with the transmission spectrum of 1586 – 1596 nm. Due to the spectra of 1591.1 and 1592.6 nm are too near to filter, an external cavity PC was applied. Through adjusting the PC, separate bright pulse and dark pulse could be realized, as shown in Fig. 5(a). The corresponding output spectrum for the bright pulse and dark pulse are shown in Fig. 5(b). Obviously, in the case of bright pulse generation, the peak at 1591.1 nm kept fixed, while peak at 1592.6 nm appeared again when the dark soliton was observed. The results showed the bright pulse and the dark pulse had different wavebands and different polarization state. Then tuned the transmission spectrum range to 1567 – 1577 nm, the bright-dark pulse pair was observed. The intensity of the bright pulse or the dark pulse could be changed, not disappeared by adjusting the external cavity PC, which indicated that only changing the polarization state was unable to separate the bright pulse and the dark pulse. Therefore, it could be concluded that the bright pulse and the dark pulse are polarized-dependent, but not linearly polarized light. As we know, the polarization effect and XPM effect may be beneficial for the formation of bright-dark pulse pair . The different wavebands of bright pulse and dark pulse may cause the XPM effect when laser propagated in fiber. Based on the experimental results, the XPM effect was regarded as the reason for the coexisting of pulse pairs [21, 22]. Generally, the saturable absorbing ability of the ReS2 is attributed to the formation of the bright pulses and the dark pulses. While the cross-phase modulation (XPM) caused by different wavebands of bright pulse and dark pulse support the coexisting of the bright-dark pulse pair. Besides, the stability of the multi-wavelength soliton pair was also observed over 2 hours by monitoring the output spectrum at the pump power of 270 mW, as shown in Fig. 6. Indeed, after one week, the soliton mode-locking operation could also run stably when the pump power was set as 270 mW, which well indicated the excellent environmental stability of ReS2.
To eliminate the possibility of mode-locked laser caused by the self-pulsing effect, the fiber laser characteristics without ReS2 were also investigated. Only CW operation was found, without any sign of mode-locking operation with the angle of PC tuned in the total range of 360° or the pump power increased from the oscillation threshold (38 mW) to the maximum (500 mW). The phenomenon verified the vital function of ReS2 in forming the bright-dark pulse pairs. The saturable absorption and nonlinear effect are attributed to the generation of bright pulse and dark pulse. The output power for CW operation as a function of pump power with a slope efficiency of 6.85% is illustrated in Fig. 4(a). In addition, the photon energy at the wavelength of 1.5 μm (0.8 eV) is much less than the bandgap of ReS2 (1.35 eV), which theoretically determines that the incident photons would not be absorbed. However, the saturable absorption properties of ReS2 at 1.5 and 2.8 μm have been presented [11, 23], as the results shown in this work. According to the previous report, the defects, such as edges or vacancies, are proposed as the reason for the change of bandgap [24, 25]. More detailed investigation will also be necessary.
In conclusion, a multi-wavelength bright-dark pulse pair has been experimentally achieved in a passively mode-locked erbium doped fiber laser based on ReS2-SA. Using the liquid exfoliation and optically driven deposition methods, the ReS2-SA was fabricated with modulation depth and saturation fluence of 6.9% and 27.5 μJ/cm2, respectively. By carefully adjusting the polarization state and pump power, the bright-dark pulse pair was observed easily with a multiwavelength emission due to the nonlinearity of ReS2. The output spectrum showed the peaks located at 1573.5, 1591.1 and 1592.6 nm, corresponding to the RF frequency separations of 744 Hz and 80 Hz. The XPM effect caused by different wavebands of bright pulse and dark pulse may support the coexisting of the bright-dark pulse pair.
National Key Research and Development Program of China (2017YFB0405204).
The authors wish to thank Dr. Kejian Yang, School of Information Science and Engineering, Shandong University for the assistance in English writing.
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