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Abstract
With many applications increasing expectations for the tunability of coherent pulses, the demand for tunable soliton fiber lasers (TSFLs) is growing. Two-dimensional ternary zirconium germanium telluride (ZrGeTe4) nanomaterials provide an opportunity to obtain high-performance TSFLs. In this research, we developed a TSFL with ZrGeTe4-based saturable absorber (SA). Employing a strategy of constant pump power, tunable soliton states were acquired. The 1st to 5th order harmonic mode-locking were obtained sequentially. The shortest pulse able to obtain was 1.05 ps. Furthermore, the soliton molecule was also obtained. The results suggest that ZrGeTe4-SA may provide an effective route for photonics research.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The mode-locking (ML) lasers have excellent characteristics such as low noise, long-term operability and easier output ultrashort pulses [1–3]. The researchers have already realized ML laser through various design solutions [4,5]. Among them, the all-fiber structure ML lasers as a simple operation solution provide a new platform for exploring next-generation ML lasers [6,7]. In recent years, due to the comprehensive development in areas nonlinear microscopic imaging, optical fiber sensing and ultrafast photonics, the demand for ML lasers that can freely switch the work state is increasing [8–10]. Therefore, the studies on TSFLs have been widely reported [11–13]. Among them, the passive scheme stands out for the advantage of repetition rate self-stabilization [14]. Meanwhile, the broadband saturable absorption, high third-order optical nonlinearity and low optical loss of SA devices provide a strong foundation for realizing SA-based TSFLs [15–19]. Moreover, SA-based TSFLs realized by using constant pump power strategy could achieve both constant average output power and switch between different soliton states [20]. This process is only realized by regulating the polarization state. It is worth mentioning that the rapid development of nanoscale material manipulation techniques has opened up more possibilities for the emergence of new SAs.
During the past years, two-dimensional (2D) nanoscale substances already demonstrated huge possibilities in producing optoelectronic devices, sensors and energy storage devices [21–23]. Since the discovery of graphene in 2004 [24,25], 2D nanomaterials like black phosphorus (BPs) [26,27], topological insulators (TIs) [28,29], transition metal dichalcogenides (TMDs) [30,31] and Xenes [32–34] have aroused much enthusiasm because of their novel physical, chemical and electronic features. In particular, their strong responding capabilities in nonlinear optics make them provide the foundation for producing optical modulation devices [35,36]. However, the exploration of 2D materials is almost entirely focused on two-element systems. The large and hard-to-adjust bandgap of most 2D binary materials is not beneficial to produce high-quality optical devices [37]. Recently, 2D ternary nanomaterials give researchers new inspiration, it has unique properties that other binary systems do not possess. For instance, the physical properties of ternary systems can be adjusted by selecting stoichiometric ratios, which can help to select the suitable compounds to match production needs for advanced electrical equipment [38]. However, using SA based on a ternary 2D material to generate ultrashort pulses is still relatively undeveloped. In this case, the study on the nonlinear optical responsiveness of ternary 2D materials presents great value.
The ternary transition metal chalcogenide ZrGeTe4 represents a new kind of semiconductor compound. The synthesis scheme of ZrGeTe4 has been reported in detail at [39]. ZrGeTe4 possesses a typical layer structure. The distance between adjacent layers of ZrGeTe4 is ∼0.82 nm [40]. The weak combination forces between the layers suggest that ZrGeTe4 with few or single layers is available through simple exfoliation methods such as liquid phase exfoliation (LPE) [42]. The researchers have theoretically and experimentally proven that ZrGeTe4 has superior properties [41]. For example, Guo et al. showed that layered ZrGeTe4 has ultrafast carrier mobility by using phonon calculations [43]. Among them, ZrGeTe4 has an average carrier mobility between graphene and MoS2 [44,45]. In addition, the environmental stability, magnetic and thermoelectric properties of ZrGeTe4 have been extensively studied [46–48]. The band gap of ZrGeTe4 has typical layer dependent properties. In general, bulk phase or few-layer ZrGeTe4 materials have indirect band gaps structure (∼0.69 eV), yet single-layer ZrGeTe4 exhibits direct band gaps (∼1.21 eV) characteristic, and medium value is 1.08 eV [49,50]. Due to the unique band gap properties, the ZrGeTe4 nanomaterial will have an enhanced ability to radiate and absorb photons, which will be useful for producing optical modulation devices. Nevertheless, the nonlinear optical responsiveness of ZrGeTe4 has been relatively poorly investigated. Therefore, increasing the efforts to study the optical performance of ZrGeTe4 is significant in the fields of optoelectronics [51], nanoelectronics [52] and ultrathin flexible devices [53] and other research areas.
In our work, we focus our research on the realization of all-fiber ZrGeTe4-SA based TSFL in 1.5 µm band. The few-layer ZrGeTe4 nanosheets were prepared by LPE. And, its morphology, elemental content, vibrational energy level, light absorption properties and light modulation properties were carefully characterized. The ∼16.29 µm diameter micro fiber was produced by the flame-heated drawing technique. Moreover, with the help of the photodeposition technique, we successfully prepared ZrGeTe4-SA. At constant pump power, different soliton states are realized in an erbium-doped fiber laser (EDFL) based on ZrGeTe4-SA only by optimizing the polarization state in the cavity. Among them, we have realized the fundamental frequency ML, 2nd, 3rd, 4th and 5th order harmonic mode-locking (HML) in turn. The pulse width of conventional solitons (CS) is 1.05 ps. In addition, we also obtained bound state solitons (BS).
2. Preparation and characterization of ZrGeTe4 materials
For better understand the used ZrGeTe4 nanomaterial, it is important to characterize it in detail. Figure 1(a) illustrates the structural features of ZrGeTe4. In each layer, the Zr atom bonded with 1 adjacent Ge atom and 7 Te atoms, a tetrahedron formed by 1 Ge atom with 3 Te atoms and 1 Zr atom [48]. The unique structure allows ZrGeTe4 to exhibit great mechanical stability and in-plane anisotropy [46]. Figure 1(b) show the surface features of the ZrGeTe4 nanomaterials documented through scanning electron microscopy (SEM, ZEISS Sigma 300) under 1 µm and 100 nm resolutions, respectively. The results show that we successfully prepared nanosized layered ZrGeTe4 materials. The energy spectrum (EDS) point-scan results of the sample are shown in Fig. 1(c). Several robust signal peaks correspond to the elements Zr, Ge and Te, and the atomic ratios satisfy 1:1:4. In addition, the elemental mapping images acquired through EDS surface scan analysis are shown in Fig. 2(d), where elements Zr, Ge and Te are represented by different colour and the regional distribution is clearly illustrated, this shows that the ZrGeTe4 nanomaterials we used are very pure.
Fig. 1. (a) Cross-sectional schematic of ZrGeTe4 crystal structure. (b) The SEM image of ZrGeTe4 at the scale 1 µm. (c) EDS result of ZrGeTe4 material. (d) Corresponding diagram of elemental maps from EDS surface scan analysis in ZrGeTe4 for Zr, Ge, and Te. (e) and (f) AFM results of ZrGeTe4.
Fig. 2. (a) The XRD result, (b) Raman shift diagram, and (c) UV-Vis-NIR absorption spectrum of ZrGeTe4.
Figure 1(e) and (f) demonstrate the thickness uniformity of the nanosheets measured by atomic force microscopy (AFM). In Fig. 1(e), positions 1, 2 and 3 were selected to analyze the thickness, the results are shown in Fig. 1(f). The statistical distribution of the heights shows that the thickness of the ZrGeTe4 nanosheets at 1, 2 and 3 is 3.4 nm, 3.1 nm and 4.3 nm, respectively. The results show that we obtained ZrGeTe4 nanosheets with uniform thickness (the corresponding number of layers is around 3-6). Since the bandgap of ZrGeTe4 is dependent on the change of layers, so we prepared ZrGeTe4 is indirect bandgap (∼0.69 eV). The narrower bandgap structure provides a basis for us to prepare broadband optical modulators based on ZrGeTe4. Meanwhile, ZrGeTe4 was analyzed by X-ray diffraction (XRD, Ultima IV). The results in Fig. 2(a) indicate that diffraction peaks were recorded at (020), (040), (060), (080), (0100), respectively. And the (0l0) plane could index all diffraction peaks [40]. Figure 2(b) shows a clear Raman spectrogram (Raman, LabRam HR Evolution) with typical Raman peaks were recorded at 87.51, 118.01, 251.43 cm-1 respectively. The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectrum for ZrGeTe4 material is displayed on Fig. 2(c). The best absorption range was between 200 and 1875nm. This shows that ZrGeTe4 material has a wide absorption range in the 1.5 µm band and reveals its broadband modulation ability.
3. Preparation and characterization of ZrGeTe4-SA
In our experiments, we obtained ZrGeTe4 nanosheets by LPE technique, and Fig. 3 shows the detailed experimental process. Firstly, 5 mg of ZrGeTe4 powder was put into 10 ml anhydrous ethanol, next they were blended carefully. Then, the mixture solution is processed for 24 hours in an ultrasonic cleaner for ensuring that the ZrGeTe4 material is completely exfoliated and dispersed. After centrifugation at 3200 rpm for 2 h, the supernatant liquid was extracted to obtain ZrGeTe4 nanosheets dispersion.
In order to reduce the effect of thermal loads on the material, the coupling method we used was tapered fiber [54,55]. According to the strength of the nonlinearity outside the micro-region is affected by the diameter and interaction length of the tapered region, the performance of SA can be controlled by tuning the parameters setting of the tapered region [56]. Figure 4(a) shows the flame-heated drawing technique we used. After flame heated central part for fiber to the melting point, the two fiber holders were stretched uniformly to both sides. Finally, the finished tapered fiber is placed on a glass plate and ready to use. After several tests, the tapered fiber in Fig. 4(b) was selected for subsequent work. The diameter of the micro-region was measured to be ∼16.29 µm.
Fig. 4. (a) Preparation device for tapered fiber. (b) Optical microscope image for a tapered fiber without ZrGeTe4 nanosheets. (c) Experimental setup of photodeposition. (d) Microscope image for tapered fiber wrapped by ZrGeTe4 nanosheets.
The photodeposition method regulates the size of the nanosheets deposited on the tapered region by controlling the time of fiber immersion in the solution and the power of the light source [57]. So, we used photodeposition techniques to deposit ZrGeTe4 nanosheets onto the tapered areas of the fiber, the home-made experimental setup was displayed on Fig. 4(c). A 1550 nm pulse laser was employed to provide optical power for the entire system. A home-made fiber limiter was used to immerse the prepared tapered fiber in ZrGeTe4 nanosheets solution. Meanwhile, the tapered fiber was connected with the light source and the optical power measurement device, respectively. After the deposition process lasted 30 minutes, we obtained tapered fibers attached ZrGeTe4 nanosheets. Figure 4(d) illustrates the morphology characteristics after deposition, the black ZrGeTe4 nanosheets uniformly wrapped micro region of the tapered fibers. This shows that we have successfully prepared a micro SA based on ZrGeTe4 nanosheets.
A balanced two-arm detector was utilized for conducting a test on the nonlinear optical characteristics for ZrGeTe4-SA. Figure 5(a) demonstrates the experimental setup, a femtosecond pulse laser provides the energy for the entire system (pulse width 353 fs, repetition frequency 28.1 MHz, central wavelength 1563.7 nm). The optical energy for optical source entered to system is regulated through an attenuator. Then, the input light is divided (reference and measurement arms) through a output coupler (OC) whose spectral ratio is 50/50. A beam of light is transmitted straight into the detector I, detector II receives another beam of light that has passed through the ZrGeTe4-SA. The data obtained from the test were processed through below nonlinear fitting formula:
where T(I) represents nonlinear transmittance, while Isat, ΔT and αns symbolize the saturation intensity, modulation depth and non-saturable losses respectively. Figure 5(b) shows nonlinear saturation curve after being fitted. As findings suggest that in the process of rising incident optical intensity, absorption efficiency of ZrGeTe4-SA gradually decreases and tends to saturation. The modulation depth of ZrGeTe4-SA was calculated to be 9.4%, while the saturation intensity was 10.87 MW/cm2. Its non-saturable losses was 20.8%. This demonstrates that the ZrGeTe4 nanosheets have outstanding nonlinear optical response ability and suitable to produce high performance optical modulation devices.Fig. 5. (a) The home-made balanced two-arm detection system. (b) The nonlinear transmission of ZrGeTe4-SA.
4. Experimental setup
The experimental setup we designed was displayed on Fig. 6. The 978 nm laser diode (LD) provides energy for the entire laser system. The 978/1550 nm WDM combines pump light into 0.3 m erbium ion doped fiber (Er80/4-125, 0.032 ps2/m). A set of polarization controllers (PCs) were utilized for managing the polarization conditions in resonant cavity. For suppressing interference of reverse light, one polarization-independent isolator (PI-ISO) was utilized. Additionally, 20 m single mode fiber (SMF/9-125, -0.022 ps2/m) is used to manage the dispersion values in the cavity. The ZrGeTe4-SA as an optical modulation device is connected to other devices in the resonant cavity. An OC with a 10:90 splitting ratio exports 10% of the cavity energy to the outside for measuring the output performance of the EDFL. The remaining energy is left to circulate and oscillate in cavity. The length and net dispersion value of the entire ring optical system were 25.65 m and -0.548 ps2 respectively. So as to analyze the performance capabilities of the EDFL, a digital oscilloscope (3054z Wavesurfer) was used for detecting the pulse transmission in the time domain state. A 0.05 nm resolution optical spectra analyzer (MS9710C) and an autocorrelator (Femtochrome FR-103XL) that enables pulse width measurements in the region of 5 fs to 90 ps were used to measure the spectral characteristics and duration of the output pulses, respectively. A 3 ghz high-speed PIN photodetector (PD-03) is used to complete the optical-to-electrical conversion and with the help of a spectrum analyzer (MXA Signal Analyzer N9020A), the output pulses were examined through radio frequency (RF) spectrum. Meanwhile, output optical power for the EDFL was received and monitored through optical power meter (GR-103A).
5. Experimental results and discussion
The high nonlinear effects in the ring cavity will induce the self-locking phenomena. To demonstrate that ZrGeTe4-SA is a key factor in ML pulse generation, we carefully examined the EDFL without SA. After freely adjusting the pump power and PCs, no pulse formation all the time. Then, ZrGeTe4-SA applied to the cavity, we detected abundant soliton phenomena at constant pump power. The results show that ZrGeTe4-SA is an indispensable factor in ML pulse creation. We demonstrate three different soliton states from the same resonant cavity and experimental results are displayed below.
5.1 Conventional soliton at fundamental frequency
During the experiment, pump power gradually enhanced to 80 mW, energy operating in the loop satisfies the oscillation condition of ML. We obtained stable self-starting ML operation. Then, by further boosting pump power, steady state of ML would not be broken. While the pump power attains 262 mW, pulse sequence, autocorrelation trajectory, spectral characteristics, and RF spectrum from ZrGeTe4-SA based EDFL were presented on Fig. 7(a-d), respectively. Figure 7(a) shows a pulse sequence that was recorded in a span of 2000ns. The time interval is 124.8 ns of adjacent pulses and the intensity of the pulses is uniform and well ordered. The inset in Fig. 7(a) shows the result from the oscilloscopic scan on pulse sequences within 5 µs, which shows that ML situation is highly stable. In order to measure pulse duration, autocorrelation trajectory was measured in this state, and Fig. 7(b) demonstrates the result. If Gaussian function is used to fit, the pulse width is measured as 1.05 ps. Time bandwidth product (TBP) calculated as 0.431, which indicates a faint chirp in ML pulse.
Fig. 7. (a) Output pulse sequence in the range of 5 µs (upper) and 2000ns (lower). (b) Autocorrelation trajectory. (c) Optical spectra. (d) RF spectra for the ML pulses, inset: broadband RF spectrum. (e) The evolution of spectrum under various pump power. (f) Performance of output power and peak power relative to pump power.
Figure 7(c) shows that CS located at 1529.95 nm exhibits the 3 dB bandwidth (Δλ) was 3.21 nm. Besides that, spectral shape is consistent with Gaussian-type distribution and the distribution of the Kelley sideband components on two sides in spectra is typical characterization for CS. Among them, energy loss in resonant cavity induces the generation of dispersive waves, and after the phase separation of the dispersive wave and soliton reaches 2π, strong interference occurs, which leads to the formation of a Kelly sideband [58]. The distribution of Kelly usually satisfies the following relationship [59]:
Figure 7(d) shows our measured RF spectrum, which scan range is 8 MHz, and the resolution bandwidth (RBW) is 300 Hz. The measurement proves that the ML has a pulse repetition on 8.01 MHz. This value corresponds well to the length of the cavity, which suggests that we have obtained ML state at fundamental frequency. In addition, the signal-to-noise ratio (SNR) reaches 72 dB, representing our ML pulses are not easily disturbed. The illustration in Fig. 7(d) displays RF spectra of the fundamental frequency ML in the scope of 200 MHz.
So as to explore the impact of pump power for ZrGeTe4-SA based EDFL, the detection was performed in the interval of 230 mW-290 mW. Figure 7(e) illustrates dynamic evolution of the spectra. As the pump power grows, the spectra central wavelength only fluctuates just in range of 0.5 nm, and the intensity of the Kelly sidebands gradually grows. The generation of these variations indicates that we have obtained high stability fundamental frequency ML in a short cavity length. In addition, there are always continuous wave components at the center of the spectrum because of the relatively high energy occupation at the center. This is very important in the CS state, continuous wave components play an major role in maintaining the soliton shape. At the same time, with increasing pump power, Fig. 7(f) demonstrates the average and peak output power of the EDFL. As the pump power grows from 230 mW to 290 mW, the 10% output power in the ring cavity increased linearly from 443.6 µW to 682.3 µW (i.e. combined output power grew ranging 4.44 mW-6.82 mW). Correspondingly, the peak power has been increased from 52.7 W to 81.1 W.
For further proving the efficiency of EDFL stability in fundamental frequency ML state, Fig. 8(a-c) displays our analysis results. Figure 8(a) exhibits the variation of the pulse sequence corresponding to different pump power. Even if changing the pump power, pulse sequence still maintains well order and uniform intensity. The results show that the EDFL is able to run steadily in the fundamental repetition frequency ML state. For proving the time stability of the ML pulses, we adjusted the pump power to 290 mW and recorded changes in spectrum every 1 minute in 60 minutes. The polarization conditions inside cavity was fixed during recording period, results are displayed on Fig. 8(b). Furthermore, the output power of the ring cavity was carefully recorded, as shown in Fig. 8(c). During 11 hours of monitoring, the output power jittered in the range of 682.3 ± 0.13 µW. As the results show, under long-term light conditions, although thermal loads continue to build up, ZrGeTe4-SA does not reduce performance. This provides the basis for stable operation of the laser.
Fig. 8. (a) Evolution of the pulse sequence relative to the pump power. (b) The spectra state relative to time. (c) The measured laser output power over 11 h.
5.2 Harmonic mode-locking
According to description for fundamental soliton condition, due to the quantization for intracavity soliton pulse power, the maximum energy of pulse appears like clamping effect (which is possible to explain via soliton area theorem) [59,60]. Thus, pump power boost will induce pulses to split [61]. At the same time, the soliton will interact with the accompanying non-soliton component, which leads to attractive and repulsive forces between the solitons [62]. Moreover, sound wave effects also play a role in the fixation of the pulse in the time-domain state. However, in our experiments, the way of generating higher-order HML pulses seems to be more consistent with the mechanism of spectral filtering effects inducing HML [63]. With a constant pump power strategy, we varied the magnitude of the birefringence effect by changing the direction of the PCs. This will have an impact over the gain bandwidth of laser. Eventually, the number of pulses will vary with the bandwidth. Currently, the combination of constant pump power and polarization control to achieve HML has demonstrated great research value in the research fields of specialty fiber characterization, supercontinuum spectroscopy, and power amplifiers [64].
In the EDFL, we acquired a set of HMLs at various repetition frequencies through managing the polarization mode within cavity without altering pump power. Firstly, angles for three paddles in PC1 are fixed in order from left to right at 1/4π, 1/2π and 3/4π respectively (according to the clockwise direction). The middle and right paddles in PC2 are set at 5/9π and 1/2π respectively. Then, the left paddle in PC2 is rotated within a range of 180°. Figure 9(a) illustrates the pulse trains with different orders of HML. In response to the change of intracavity polarization state, the HML order increases, the time interval of neighboring pulses becomes shorter, and the corresponding repetition frequency increases. Pulse separation for the 1st, 2nd, 3rd, 4th and 5th order HML is 124.8 ns, 62.3 ns, 42.1 ns, 30.2 ns and 24.3 ns respectively. It is worth noting that after each high-order HML experienced the change of polarization state, the pulse trains will jitter slightly and then will maintain a stable order and intensity. In order to illustrate the stability of each order HML, the RBW value was set to 300 Hz, the corresponding RF spectrum was measured, and the results are shown in Fig. 9(b). The SNR corresponding to ML pulses with repetition frequencies of 8.01 MHz, 16.06 MHz, 23.75 MHz, 33.15 MHz and 41.20 MHz are 75 dB, 75 dB, 80 dB, 55 dB and 85 dB respectively.
Fig. 9. (a) The pulse trains at different orders of HML. (b) The RF spectrum of HML at different repetition frequency.
Due to the presence of super mode noise, this will produce non-negligible effects on the stability of HML. Therefore, the supermode suppression ratio (SMSR) is an critical indicators in the laser. Figure 10(a) displays the variation of SNR and SMSR along the increases of HML order. The value of SNR increases from 75 dB and remains stable. However, when the 4th order harmonic mode-locking is obtained, the pulse jitter affects the whole trend of SNR. The value of SMSR is always higher than 50 dB. This indicates that our designed high repetition frequency EDFL based on ZrGeTe4-SA has a good stability. Figure 10(b) shows the spectral evolution after the change of polarization state. We found that the sidebands were always situated on opposite sides of the spectrum no matter how repetition frequency varied, this is typical behavior of HML.
Fig. 10. (a) The dependency of SNR and SMSR on harmonic order. (b) Evolution of spectrum at different orders of HML. (c) Dependency of spectrum 3 dB bandwidth and central wavelength on repetition frequency. (d) Dependence of single pulse energy and pulse width on harmonic order.
Furthermore, in order to fully analyze the variation of the spectrum, we have carefully recorded the 3 dB bandwidth and the central wavelength of the spectra associated with the repetition frequency on Fig. 10(c). After increasing the repetition frequency from 8.01 MHz to 41.20 MHz, 3 dB spectra width reduced down from 3.21 nm to 2.26 nm, yet the central wavelength almost no shift occurs. The spectral bandwidth becomes narrower because of the presence of spectral filter effects. Figure 10(d) demonstrates that the effect of changing the order of HML on the single pulse energy and pulse duration. During variation from 1st to 5th order HML, the single pulse energy is reduced from 67.67 pJ to 13.16 pJ. With constant pump power, the decrease in maximum single pulse energy drives the HML operation. The corresponding pulse width was boosted from 1.05 ps to 1.77 ps.
5.3 Bound state solitons
After a single optical soliton split into multiple solitons, the attractiveness and repulsion between solitons will result in establishment of a new balance state. Then, the multiple solitons will travel with the same phase relationship and spread speed [65]. Eventually, the multiple solitons will continue to travel along the fiber in a bound state, which is called BS. In recent years, with the research on BS getting more advanced, its applications in areas such as soliton dynamics analysis and encrypted information transmission have become more widespread [66].
In this experiment, pump power was controlled as 262 mW, we obtained multiple soliton state with fixed phase relations and good order by managing the polarization condition inside cavity. Figure 11 displays oscilloscope trajectory and spectral variation of the multiple soliton beam. With the occurrence of intracavity polarization mode changes, the energy traveling inside cavity breaks limit on single soliton energy and single soliton occurs significantly splitting and shifting. Figure 11(a1) shows the oscilloscope trace in the starting state and the corresponding spectrum is displayed in Fig. 11(a2). By slightly adjusting the direction of the PC, a low intensity sub-pulse can be observed next to the main pulse in Fig. 11(b1). At this moment, the soliton splits and a slight modulation can also be found in the corresponding spectrum Fig. 11(b2). The inset in Fig. 11(b2) shows the details of this modulation.
Fig. 11. (a1), (b1), and (c1) are the oscilloscope trace evolution processes of the BS; their corresponding spectrum evolutions are (a2), (b2), and (c2) respectively.
After we adjusted the PC along the same direction, the pulse separation continued to vary and eventually stabilized, the results are shown in Fig. 11(c1). As seen from the results in Fig. 11(c2), the spectrum in this state is symmetrically distributed relative to the center wavelength and shows a distinct periodic modulation. The characteristics of the spectral distribution are consistent with a two-soliton bound state when the phase difference is close to π [67]. The inset in Fig. 11 (c2) illustrates that the spectral center wavelength is 1529.4 nm and the distance between the spectral peaks is 1.84 nm, corresponding to the spectral modulation period. Since no autocorrelator was used to actually measure the pulse separation time of the soliton, we were only able to theoretically predict a pulse separation time of 4.24 ps based on the spectral modulation frequency theory. In the future, we will solve the problem of instruments and further explore more details of the bound state soliton output from ZrGeTe4-SA based ring lasers.
Table 1 summarizes the results of mode-locked fiber lasers based on SAs of different 2D materials [68–74]. This indicates that our designed ZrGeTe4-SA based TSFL is effective. In addition, the comprehensive performance of ZrGeTe4-SA is competitive compared to the widely reported SAs.
Table 1. Mode-locking fiber lasers using different nanomaterials as SAsa
6. Conclusion
In this research, we realized ZrGeTe4-SA based TSFL. LPE, photodeposition and tapered fibers were used for SA preparation. The modulation depth as well as saturation intensity for the ZrGeTe4-SA showed 9.4% and 10.87 MW/cm2, respectively. In the TSFL based on ZrGeTe4-SA, the separation of soliton pulses is achieved using a strategy of constant pump power and polarization control. Stable CS and multiple higher-order HML were obtained. Among them, the CS showed the repetition frequency of 8.01 MHz, the narrowest pulse duration as 1.05 ps and maximum peak power as 81 W. Meanwhile, the highest repetition frequency which the TSFL can output is 41.20 MHz, corresponding to 5th order HML. In addition, the shift of intracavity polarization conditions also induces formation of BS. The modulation period is 1.84 nm. This indicates that the TSFL based on ZrGeTe4-SA may be a new approach to the research of next generation lasers.
Funding
National Natural Science Foundation of China (11904213); Natural Science Foundation of Shandong Province (ZR2020MA087, ZR2020QA066, ZR2021QE178); Natural Science Basic Research Program of Shaanxi Province (2022JQ-075).
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 authors upon reasonable request.
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