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Abstract
We proposed an erbium-doped fiber laser mode-locked with a MoxW1-xTe2-based nonlinear optical modulator for the first time to our best knowledge. This fiber laser can deliver bright pulses, bright-dark pulse pairs, dark pulses, bright-dark-bright pulses, and dark-dark-bright pulses. The modulation depth and saturation intensity of the MoxW1-xTe2-based saturable absorber were about 7.8% and 8.6 MW/cm2, respectively. When 10% of the laser in the cavity was output, conventional soliton pulses with central wavelength of 1560.1 nm can be obtained in the cavity. When 70% of the laser was output, dual-wavelength domain-wall dark pulses appeared in the laser cavity. This experiment revealed that an appropriate increase in the ratio of output energy can improve the chance of dark pulses in fiber lasers. The mode-locking states in this fiber laser can evolve with each other between bright pulses, bright-dark pulse pairs and dark pulses by adjusting the polarization controller. The results indicated that the MoxW1-xTe2 can be used to make modulators for generating dark pulses. Furthermore, our work will be of great help to improve the chance of the generation of dark pulse in fiber lasers.
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1. Introduction
In photonic technology, pulsed lasers play an important role in a variety of applications. With the advancement of laser technology, pulsed lasers have been widely used in fiber optic communication, fundamental physics, micromachining, medical surgery, and military defense [1–5]. Generally, the pulse referred to a bright pulse, i.e., a sudden and significant increase in light intensity against a background of weak and stable continuous light waves [6,7]. However, there was not only bright pulses, but also dark pulses. Dark pulse was the opposite concept to bright pulse, i.e., a sudden and significant drop in light intensity against a background of stable continuous waves [8–11]. In comparison with bright pulses, dark pulses possessed lower noise, lower loss and faster propagation speed in optical fibers, which made them promising for signal processing and long-distance communication [12,13]. In 2009, dark pulses were directly obtained in fiber lasers for the first time [14]. Subsequently, many studies were conducted on dark-pulse fiber lasers, and the obtained dark pulses were classified into three categories: nonlinear Schrödinger equation (NLSE) dark pulse, cubic-quintic nonlinear Schrödinger equation (CQNLSE) dark pulse, and domain-walled (DW) dark pulse [15,16]. These three categories of dark pulses were similar in nature, and their classification was based on the differences in the structure of the cavities that generated them. NLSE dark pulse was generated in a normal dispersion cavity, so the NLSE dark-pulse erbium-doped fiber (EDF) laser usually contains a dispersion compensating fiber (DCF) or dispersion shifted fiber (DSF) [17,18]. Highly nonlinear fiber or photonic crystal fiber were typically found in CQNLSE dark-pulse fiber lasers because the formation of CQNLSE dark pulse was dependent on higher-order nonlinearities [19,20]. The formation of DW dark pulses did not rely on dispersion management or higher-order nonlinearity, but rather on the interaction between lasers of different wavelengths [21–23]. Moreover, saturable absorbers (SAs) based on two-dimensional (2D) materials were often added to the fiber lasers to assist in the generation of DW dark pulses [24,25].
Since the emergence of graphene in 2004, 2D nanomaterials have attracted significant interest in basic and applied research due to their attractive structures and remarkable optoelectronic properties [26,27]. Many 2D materials, like transition metal chalcogenides (TMCs), topological insulator, black phosphorus, etc., have been found to have ultrafast saturable absorption properties and have been incorporated into fiber lasers to obtain ultrashort pulses [28–33]. Most of these materials can be used to generate not only normal bright pulses but also dark pulses [34]. Ahmad et al. demonstrated dual-wavelength DW dark pulse operation based on MoS2 in an Ytterbium-doped fiber laser [35]. Based on WS2, Liu et al. obtained dark pulse in an EDF laser with center wavelength of 1530 nm and repetition rate of 116.5 MHz [36]. In 2020, Wang et al. acquired bright-dark pulse pairs in a thulium-holmium-doped fiber ring laser based on MoSe2 as a SA [37]. MoS2, WS2, MoSe2 belongs to TMCs and their properties and applications have been studied in detail. However, the research on ternary transition metal chalcogenides (TTMCs) in the field of photonics is still in its early stage. The general chemical formula of TTMCs is MxN1-xX2, where M and N are two different transition metal elements, X is one of S, Se or Te, and x = 0 to 1. M and N can be any combination of two transition metals, and the mass ratio of both can be controlled, resulting in more flexible performance and applications of TTMCs. In 2021, liu et al. investigated the nonlinear optical properties of WxNb1-xSe2 and RexNb1-xS2 and their applications in fiber lasers [38,39]. In their experiment, mode-locked and Q-switched pulses were obtained based on WxNb1-xSe2, and conventional solitons and bound-state solitons were obtained based on RexNb1-xS2. MoxW1-xTe2 is one member of the TTMCs, whose nonlinear optical properties and applications in ultrafast fiber lasers have not been studied so far.
In this paper, an EDF laser with a MoxW1-xTe2-based nonlinear optical modulator was fabricated for the first time, which can deliver bright pulses, bright-dark pulse pairs, dark pulses, bright-dark-bright pulses, and dark-dark-bright pulses. This MoxW1-xTe2-based modulator has a modulation depth of 7.8% and a saturation intensity of 8.6 MW/cm2. When 10% of the laser in the cavity was output, conventional soliton pulses with central wavelength of 1560.1 nm were formed in the cavity. When 70% of the laser in the cavity was output, dual-wavelength DW dark pulses were obtained. It was revealed experimentally that an appropriate increase in the ratio of output energy can improve the chance of dark pulses in fiber lasers. The mode-locking states in the cavity can evolve with each other between bright pulses, bright-dark pulse pairs and dark pulses by adjusting the PC. The results indicate that the MoxW1-xTe2 could be developed as an alternative modulator for the generation dark pulses. Furthermore, our work will be of great help for the development of the application of TTMCs in ultrafast optics and for the improvement of dark-pulse fiber lasers.
2. Experiments
2.1 Characteristics of the MoxW1-xTe2
The MoxW1-xTe2 crystal had a typical layered structure, which was evident from the scanning electron microscope (SEM) image of the crystalline powder, as shown in Fig. 1(a). Since the crystals had a layered structure, the material can be peeled into few-layered 2D nanosheets. Energy-dispersive X-ray spectroscopy (EDS) was applied to determine the quantity of each element in the MoxW1-xTe2 material. As the EDS spectrum displayed in Fig. 1(b), distinct peaks of Mo, W, and Te can be seen, and the atomic ratio of Mo, W, and Te was estimated to be about 0.5:0.5:2, which indicated that the value of x was equal to 0.5. Raman spectrum measurement were further used to characterize the MoxW1-xTe2 crystal powder by a 633 nm laser at room temperature. There were six peaks centered at 78.6, 89.9, 110.1, 131.8, 162.1 and 210.3 cm−1, respectively, as shown in Fig. 1(c). The peaks centered at 78.6 and 162.1 cm−1 associated with the Ag and Bg modes of β-MoTe2 crystals, and the remaining four peaks were ascribed to the A1 and A2 modes of Td-WTe2 [40]. An X-ray diffractometer was utilized to perform X-ray diffraction (XRD) measurement on the MoxW1-xTe2 crystals. The XRD pattern shown in the Fig. 1(d) were consistent with previous research, which indicated that the material was in the 1T′+Td two-phase state [41].
Fig. 1. (a) SEM image, (b) EDS spectrum, (c) Raman spectrum and (d) XRD pattern of the MoxW1-xTe2 crystal powder.
MoxW1-xTe2 nanosheets obtained from powder by using liquid phase exfoliation technology. Firstly, 10 mg of the powder was dispersed in 20 mL of alcohol. Secondly, the mixture solution was sonicated for 10 h. Thirdly, uniformly sized MoxW1-xTe2 nanosheets can be acquired by centrifugation of the dispersion solution at 10,000 r/min for 5 min. The thickness of nanosheets was measured by an atomic force microscope (AFM), as illustrated in Fig. 2(a). As presented in Fig. 2(b), the thicknesses of the nanosheets at these two locations were ∼1.5 nm and ∼1.8 nm, respectively, both of which are less than 2 nm. The transmission electron microscope (TEM) image of the MoxW1-xTe2 nanosheets at resolutions of 50 nm was exhibited in Fig. 2(c), which indicated the nanosheets has obvious crystal lattice structure. We also captured the high-resolution TEM image at resolutions 2 nm, as shown in Fig. 2(d). As seen in the figure, the lattice spacing was about 0.23 nm.
Fig. 2. (a) AFM image of MoxW1-xTe2 nanosheets; (b) the thickness of the nanosheets; TEM image in a (c) 50 nm and (d) 2 nm scale.
The nanosheet dispersion of MoxW1-xTe2 was mixed with 5 wt% polyvinyl alcohol (PVA) solution in a 2:3 volume ratio. Ultrasonic the mixture for another 3 hours to prepare a homogeneous MoxW1-xTe2-PVA dispersion. Spread the mixture evenly on a flat and clean substrate and leave it to dry at room temperature for 48 hours to obtain the MoxW1-xTe2-PVA film. The film was next cut into small pieces of 1.5 × 1.5 mm2. A film was then sandwiched between the two fiber ends to create a modulator that can be used in a fiber laser. The nonlinear optical characteristics of the modulator was studied by balanced twin-detector technique with a home-made femtosecond pulsed light source. The central wavelength, pulse width and repetition frequency of the light source were 1564.6 nm, 691 fs and 20 MHz, respectively. Figure 3 shown the nonlinear transmittance curve of the MoxW1-xTe2 based modulator. After fitting the curve by the well-known two-level saturable absorption model. The modulation depth (ΔT), saturable intensity (Isat), and and nonsaturable losses (αns) were given as 7.8%, 8.6 MW/cm2 and 12.7%, respectively.
2.2 Setup of the fiber laser
The structure of the ring laser cavity was demonstrated in Fig. 4. A 30-cm EDF (Er 110) was utilized as the gain medium of the fiber laser, whose group velocity dispersion (GVD) at 1550 nm was 58.63 ps2/km. The EDF was pump by a 976 nm laser diode (LD) with maximum power of 600 mW through a 980/1550 nm wavelength division multiplexer (WDM). The unidirectional operation of the entire annular cavity was ensured by a polarization-independent isolator (PI-ISO). A polarization controller (PC) was used to adjust the polarization state of the laser. A portion of the laser light in the cavity will be output through an optical coupler (OC). The MoxW1-xTe2 based SA (MWT-SA) was inserted between PC and OC, which acted as a modulator to enable the laser cavity to obtain mode-locked pulses. An optical spectrum analyzer (Yokogawa AQ6370B), an autocorrelator (FR-103XL), an oscilloscope (Wavesufer 3054Z), and a radio-frequency (RF) analyzer (R&S FPC1000) were utilized to monitor the output laser.
3. Results and discussion
3.1 Conventional soliton pulse
When a 10:90 OC was used in the laser, the total length of the device’s pigtail and additional single mode fiber (SMF) was 35.2 meters. The GVD of SMF was −21.68 ps2/km, and the net dispersion of the cavity can be calculated as −0.745 ps2. Continuous wave (CW) was first output from the cavity when the pump power gradually increased to 28 mW, whose spectrum was displayed in Fig. 5(a). Before the pump power increased to 89 mW, no pulse was formed in this fiber laser no matter how the PC was adjusted. Once the pump power increased to over 89 mW, conventional soliton pulse can be obtained by adjusting the PC properly. The optical spectrum, pulse train and RF spectrum of this conventional soliton were shown in Fig. 5. From the Fig. 5(b), it can be seen that its central wavelength was 1560.1 nm, and its 3-dB bandwidth was 4.7 nm. Multiple sets of sidebands were symmetrically distributed on both sides of the spectrum, which was a typical feature of conventional solitons. The sidebands of spectrum stemmed from the constructive interference between the soliton and dispersive waves. Given that the soliton may have a slight chirp, the pulse width can be estimated from the time-bandwidth product to be slightly larger than 543.7 fs. The oscilloscope trace in Fig. 5(c) shown that the separation between adjacent pulses was ∼172.8 ns, which corresponded exactly to the cavity length. The amplitude of the pulses remained almost unchanged, indicating that the conventional-soliton mode-locking state was very stable. The pulse intensity did not start from 0 because there was some CW in the cavity at this time. As displayed in Fig. 5(d), the basic repetition frequency of the pulses was 5.79 MHz. The RF spectrum given a signal-to-background ratio of 61 dB, which further indicated that the stability of the output pulse was very good.
3.2 Evolution between bright, bright-dark pulse and dark pulse
In order to obtain dark pulse or bright-dark pulse pairs, the cavity structure of the laser have been adjusted several times, such as changing the length of the SMF or EDF and varying the ratio of the output light. When the EDF, SMF and total cavity length were kept constant and the ratio of the output laser was gradually increased, the pulses that can be obtained in the fiber laser were summarized in Table 1. It can be seen that increasing the ratio of output light to total energy in the cavity was beneficial to the formation of dark pulses. A large output ratio tended to result in a very strong CW generated in the laser, which was favorable to the formation of dark pulses [42].
Table 1. Pulses obtained in the EDF laser with different output ratio
When we use a 30:70 OC with 70% of the laser output from the cavity, the bright-dark pulse pairs were obtained by adjusting the pump power and the state of PC. Figure 6(a) shown the oscilloscope trace of the bright-dark pulse pairs. The time interval between pulses was 175.4 ns, which indicated that the exact cavity length of this fiber laser at this time was 36 m. From the pulse sequence, it can be seen that the intensity of the dark pulse did not reach zero, because this dark pulse was actually a “gray” soliton, and its blackness parameter |B| was less than one [43]. The oscilloscope traces of a single bright-dark pulse pair were displayed in Fig. 6(b). Due to the limitation of the bandwidth of oscilloscope, this was not the true trace of the pulse. But it can be seen that the width of the bright pulse was larger than the width of the dark pulse, which was resulted from the difference in evolution mechanisms and transmission properties of the two types of pulses [16][42]. Figure 6(c) depicted the spectra of the bright-dark pulse pairs, which have two distinct intensity peaks located at 1562.5 nm and 1563.1 nm, respectively. The characteristics of the spectrum indicated that the dark pulses output from the cavity were typical dual-wavelength DW dark pulses [16]. Unlike conventional soliton pulses, the spectrum of the bright-dark pulse pairs have no sidebands. The large pulse width and narrow spectral width of the bright-dark pulses may be responsible for the absence of spectral sidebands [44–46]. The RF spectrum of the bright-dark pulse pairs was shown in Fig. 6(d). RF spectrum has only one intensity peak centered at 5.7 MHz with a signal-to-background ratio of ∼66.6 dB. Generally, dual-wavelength lasers have two pulse trains oscillating in the cavity, resulting in an RF spectrum with two main peaks. However, the two optical components of the DW dark pulses have the same repetition frequency, which confirmed that the typical dual-wavelength DW dark pulse has generated [35]. MWT-SA acted as a polarization modulator in the laser during the generation of the dark pulses [15][20]
Fig. 6. Characteristics of the bright-dark pulse pairs. (a) Oscilloscope trace; (b) single bright-dark pulse pairs; (c) optical spectrum; (d) RF spectrum.
By carefully adjusting PC, the bright-dark pulse pair can be converted into a bright pulse or evolve into a dark pulse. Regulation of the PC can change the state of those initial “seed dips” which can evolve into dark pulses. During the evolution process, bright-dark pulse pair with different amplitude ratios can be obtained, as shown in Fig. 7(a). The pulse sequence displayed in Fig. 7(a), amplitude ratios of the bright and dark pulses were about 1:0, 0.75:0.25, 0.44:0.56, 0.34:0.66 and 0:1, respectively. The trajectory of the bright pulse does not start from zero when the cavity was in the bright pulse state, indicating that there was a strong direct-current component in the cavity at this time. The darkness of the dark pulses in these states are all less than one, which can be caused by the initial “seed dips”, overly strong CW background level and limited bandwidth of oscilloscope [14][47]. Figure 7(b) shown the output power of the fiber laser in the three states of bright pulse, dark pulse and bright-dark pulse with the variation of pump power. The output power increased almost linearly in all three states. It can be seen from the figure that the output power of the fiber laser at the same pump power in the bright-dark pulse pairs was slightly less than its output power in the bright or dark pulses.
Fig. 7. (a) The evolution of the bright-dark pulse pair; (b) output power of the fiber laser in three states.
3.3 Bright-dark-bright pulses and dark-dark-bright pulses
The output pulse of the fiber laser can change from bright-dark pulse to bright-dark-bright pulse when we adjust the PC and also varied the pump power. The oscilloscope trace of the bright-dark-bright pulse was exhibited in Fig. 8(a). The time interval between two adjacent dark pulses was 175.4 ns, and the time interval between the corresponding bright pulses within the adjacent group was also 175.4 ns. At this time, the spectrum of the output pulse has three intensity peaks located at 1562.56, 1563.25 and 1563.9 nm respectively, as shown in Fig. 8(b). The presence of triple wavelengths was attributed to the comb filtering effect, which was caused by the polarization-dependent loss attributed to the PC and birefringence of the SMF [48]. In addition, the cavity can also obtain dark-dark-bright pulses, whose pulse sequence was shown in Fig. 8(c). Both states have very similar spectral shapes, with only a slight difference in intensity. The RF spectra of the bright-dark-bright pulse and dark-dark-bright pulse were displayed in Fig. 8(d). The two states have the same fundamental frequency of 5.7 MHz and the signal-to-background ratio reached 57.4 and 61.2 dB respectively, which indicated that the stability of both states was very well. When we removed the MoxW1-xTe2 based modulator from the cavity, no matter how we adjust the PC and pump power there was no pulse output from the fiber laser, which means that all pulses were obtained thanks to this modulator.
Fig. 8. (a) Pulse train of the bright-dark-bright pulses; (b) optical spectra; (c) pulse train of the dark-dark-bright pulses; (d) RF spectrum.
4. Conclusion
A MoxW1-xTe2-based nonlinear optical modulator was fabricated, with the help of which bright pulses, bright-dark pulse pairs, dark pulses, bright-dark-bright pulses, and dark-dark-bright pulses were obtained in a fiber laser. The modulation depth and saturation intensity of the MWT-SA were about 7.8% and 8.6 MW/cm2, respectively. Conventional soliton pulses with central wavelength of 1560.1 nm were formed in the cavity when 10% of the laser in the cavity was output. Dual-wavelength DW dark pulses were obtained at 70% of the laser output from the cavity. It was revealed experimentally that an appropriate increase in the ratio of output energy can improve the chance of dark pulses in fiber lasers. The mode-locking states in the cavity can evolve with each other between bright pulses, bright-dark pulse pairs and dark pulses by adjusting the PC. The results indicate that the MoxW1-xTe2 can be used to make modulators for generating dark pulses. Furthermore, our research will enhance the formation of dark pulses in fiber lasers and promote the development of dark pulse fiber lasers.
Funding
National Natural Science Foundation of China (11904213); Natural Science Foundation of Shandong Province (ZR2020MA087, ZR2020QA066, ZR2021QA050).
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 maybe obtained from the authors upon reasonable request.
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