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Abstract
Low-threshold and narrow-pulse-width Q-switched optical pulses are obtained using a tapered fiber-based WTe2-graphene saturable absorber (SA) in an erbium-doped fiber laser (EDFL) for the first time. Benefiting from the low unsaturated loss of graphene, the Q-switching threshold (25.8 mW) is an order of magnitude lower than that using SA with WTe2 only. Meanwhile, owing to the high carrier mobility of WTe2, the minimum pulse width (1.2 µs) is the narrowest among the graphene-based fiber lasers. The results indicate the potential of the WTe2-graphene SA in future application and development of laser pulses.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
27 September 2022: Typographical corrections were made to the author list.
1. Introduction
Compared with mode-locked ultrashort pulses, microsecond or even nanosecond-scale optical pulses with higher pulse energy can be applied in some specific fields, such as special biological medicine, laser ranging, nonlinear frequency conversion and optical time domain reflectometer [1]. The Q-switching technique is an effective method to generate this kind of pulses with µs/ns duration [2].
At present, active Q-switching technology requires the help of an external modulator to achieve Q-switching pulses, therefore the SA-based passive Q-switching technology with simple structure, low cost and easy integration is used more and more widely in lasers to realize the ms/ns optical pulses [3–11]. Normally, SAs are divided into two categories, artificial SAs and real SAs. Since artificial SAs based on the nonlinear characteristics of optical fibers are more sensitive to environmental disturbances, Q-switching is usually achieved by using real SAs with high stability.
Real SAs include semiconductor saturable absorber mirror (SESAM) and nano materials. SESAMs were initially used in Q-switching technology, and were gradually commercialized after several years of continuous development. This technology allows precise control of absorption wavelength, saturation threshold power, modulation depth and other parameters. However, there are still some difficulties to be overcome, such as lower damage threshold and high cost [12]. As one-dimensional material of carbon, carbon nanotube (CNT) highlights the higher damage threshold power, easier preparation and lower cost compared with SESAM, but the scattering loss for the laser pulse is still high [13,14].
In order to achieve better performance of fiber lasers, various materials have been used to prepare the SA, such as graphene [15], topological insulator (TI) [16,17], transition metal dichalcogenides (TMDs) [18,19], Black Phosphorus (BP) [20,21], Topological semimetal [22], MXene [23–25], Transition metal monochalcogenides (TMMCs) [26], Skutterudites [27] and so on. Among these materials, graphene has excellent saturable absorbing properties, such as adjustable modulation depth depending on the number of layers, high conversion efficiency and high damage threshold, which is an ideal material for the preparation of the SA in fiber lasers [28–31]. In addition, graphene has low unsaturated loss, which can effectively reduce the Q-switching threshold of fiber lasers. For instance, Luo et al. achieved a Q-switched EDFL based on graphene with a low pump threshold of 6.5 mW at 974 nm in 2010 [4]. In 2014, Rosdin et al. demonstrated a stable Q-switched pulse train using graphene polyvinyl alcohol (PVA) thin film as the SA at 1560 nm with a low-threshold pump power of 7.4 mW and a pulse width of 3.56 µs [32].
Recently, as a new two-dimensional layered material with direct band gap, TMD has been found to have better electronic band structure and band gap than graphene, which makes it easier to realize electronic transition and recombination process [33]. Therefore, the lifespan of the corresponding carrier is relatively short in this material, implying the advantages of the generation of ultrashort pulses [34]. The element composition form of TMDs can be expressed as MX2, where M is a transition metal element of group IV (e.g. molybdenum, tungsten, titanium, etc.), and X is a chalcogenide element of group V or VI (e.g. sulfur, selenium, tellurium, etc.). In the family of TMDs, the combination of different elements leads to various electrical properties. For example, WS2 and MoS2 are semiconductor materials while WTe2 is a semimetal material [35–37]. In 2014, Luo et al. proposed a Q-switched doped-fiber laser utilizing the MoS2 film SA in a wide wavelength range with a maximum pulse energy of 1 µJ and a minimum pulse duration of 1.76 µs [38]. In 2015, Kassani prepared a SA with WS2 nano-sheets in a side-polished fiber by liquid phase exfoliation method (LPE), and obtained Q-switched laser pulses with a minimum pulse-width of 0.71 µs [39]. Tungsten ditelluride (WTe2) is also verified to have good electronic properties with narrow band gap and high carrier mobility, which is suitable for near-infrared systems such as photodetectors and ultra-fast optics devices [40–43]. According to previous reports [44–47], the fastest carrier mobility of WTe2 is much higher than that of graphene, which means that pulse width generated in fiber lasers based on a WTe2 SA can be narrower. However, the Q-switching threshold of WTe2 is relatively high. In 2018, Ko et al. realized a passive Q-switching based on a bulk-like WTe2 SA with the narrowest pulse width of 1 µs and a threshold power of 110 mW [48]. In the second year, Liu et al. fabricated a tapered fiber structure of WTe2 SA for application in a Q-switched fiber laser at 1.5 µm with a threshold power of 212 mW [49]. As mentioned above, the unsaturated loss of graphene is relatively low, which can effectively reduce the Q-switching threshold. Therefore, one can consider whether to use a mixture of the two materials to realize Q-switched optical pulses with low threshold power and narrow pulse width. So far, this possibility has not been explored yet.
In this work, the WTe2-graphene composite was deposited on a tapered fiber by optical deposition method. The Q-switched threshold power and pulse width were effectively reduced by combining these two kinds of two-dimensional (2D) materials, and highly stable output pulses were obtained in the Q-switched EDFL. The results indicate that the WTe2-graphene SA not only has simple fabrication process and low cost, but also can give full play to the unique advantages of the two materials, is expected to have attractive application prospects in ultra-fast fiber lasers.
2. Fabrication and characterization of the WTe2-graphene SA
2.1 Preparation of the WTe2-graphene SA
To prepare the WTe2-graphene composite, firstly, the raw powders of graphene (50 mg) and WTe2 (50 mg) were uniformly mixed. Then the mixture was added into alcohol (20 ml) and was stirred for over 30 minutes until it was fully dissolved. Next, the solution was placed into an ultrasonic device for 3 hours to ensure the uniform dispersion of WTe2 and graphene, thus completing the preparation of the composite. In the WTe2-graphene composite, graphene and WTe2 nano-particles form a close-packed structure, which prevents the agglomeration of the graphene. The two-dimensional graphene bridges between nano-particles and provides an effective optical transmission channel for the composite. In order to analyze the distribution of the WTe2 and graphene nano-particles in the composite, the WTe2-graphene film was measured using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Figure 1(a) shows the powder XRD pattern of the WTe2-graphene film at room temperature. The apparent peak at the angle of 26.6° corresponds to the characteristic peak of the graphene for (002) [50]. Besides, several diffraction peaks are also observed at 12.6°, 25.4°, 35.1°, 38.4°, 47.4°, 51.9°, 54.8° and 66.5°, which are consistent with the diffraction of the planes of WTe2 for (002), (004), (113), (024), (017), (008), (133) and (0010) respectively. No impurity phase is observed in the XRD power pattern. Figure 1(b) and 1(c) show the SEM images of WTe2-graphene composite under different resolutions. The obvious flake structure of the graphene and flower-like nano-structure of WTe2 can be observed, which are consistent with relevant researches [51–55].
Fig. 1. Physical characterization of the WTe2 -graphene SA. (a) X-ray diffraction pattern of the WTe2 -graphene film, (b) SEM image of the WTe2 -graphene film at resolutions of 20 µm, (c) SEM image of the WTe2 -graphene film at resolutions of 2 µm.
To further confirm the existence of graphene and WTe2, we used transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) to deeply analyze the morphology and structural characteristics of the two materials. TEM images of graphene and WTe2 nanosheets at resolutions of 100 nm are shown in Fig. 2(a) and (b), respectively. There are obvious wrinkles and folds in graphene nanosheets, which have better flexibility than WTe2 nanosheets. In addition, HRTEM images present the perfect crystalline structures of graphene and WTe2 nanosheets. The hexagonal arrangement of carbon rings in graphene nanosheets can be identified from the enlarged region of Fig. 2(c), and the C-C bond length is measured to be 0.194 nm which is close to the value reported by Tapasztó et al. [56]. Figure 2(d) shows that the chains of W atoms (red circles) form the ideal distorted hexagonal nets, and the bond spacing of W atoms in different directions are 0.282 nm, 0.352 nm and 0.441 nm, respectively, which are consistent with the data from the work of Ali et al. [57].
Fig. 2. Morphology and structural characterization of the WTe2 -graphene SA. (a) TEM image of graphene nanosheets at resolutions of 100 nm, (b) TEM image of WTe2 nanosheets at resolutions of 100 nm, (c) HRTEM image of graphene nanosheets at resolutions of 5 nm, (d) HRTEM image of WTe2 nanosheets at resolutions of 5 nm.
Thus, the above results confirm the existence of both graphene and WTe2 in the composite material.
The WTe2-graphene SA was successfully prepared by optical deposition on the tapered fiber, which was a simple and effective method to deposit various materials on the substrate (tapered fiber) under the action of light. During the optical deposition process of WTe2-graphene SA, the following factors were strictly controlled, including the loss of optical fiber, the concentration of deposition material, laser intensity, laser mode and deposition time. For optical deposition, an effective fused zone with a length of 0.5 cm and a waist diameter of 10 µm was obtained by stretching a single-mode fibers (SMF-28e) on a fused biconical taper machine. Subsequently, the prepared solution was deposited on the tapered area with the action of the evanescent field by adding a beam of 15 W light. The deposition process lasted for 5 minutes to allow the material nanoparticles to be fully absorbed around the tapered region. Thus, an effective optical deposition film was obtained. Finally, the SA was dried at room temperature over 24 hours to form a well-distributed WTe2-graphene film on the tapered fiber.
2.2 Nonlinear optical characterization of the WTe2-graphene SA
The nonlinear absorption property of the WTe2-graphene SA was measured by the balanced twin detector system. The experimental system is sketched in Fig. 3(a), which consists of a nonlinear polarization rotation (NPR) mode-locked fiber laser (MLFL), an attenuator, a 3 dB fiber coupler and two optical power meters. The NPR MLFL generates stable laser pulses with a pulse width of 1.2 ps and a repetition rate of 12.2 MHz at the center wavelength of 1560 nm. The nonlinear transmission curve of the WTe2-graphene SA is displayed in Fig. 3(b).
Fig. 3. Nonlinear optical characterization of the WTe2-graphene SA. (a) Balanced twin-detector measurement system to characterize the feature of the WTe2-graphene SA; (b) The nonlinear transmission curve of WTe2 -graphene SA.
The properties of a SA are generally described by some important parameters, such as modulation depth, saturation intensity and unsaturated loss. The nonlinear transmission curve can be fitted by the following formula [58],
where $T(I )$ is the transmission function of the SA, $\Delta T$ is the modulation depth, I is the input peak intensity, ${I_{sat}}$ is the saturation intensity, and ${T_{ns}}$ is the non-saturable loss. According to the fitting results, the WTe2-graphene SA has good nonlinear saturable absorption characteristics with a saturation intensity of 3.5817 MW/cm2, a modulation depth of 3.78%, and a non-saturable loss of 31.44%, respectively. The low non-saturable loss indicates the high optical conversion efficiency of the WTe2-graphene SA.In order to determine which characteristic of the SA was involved in the Q-switching, we further tested that the polarization-dependent loss (PDL) of the SA was 0.1 dB, including the PDL of the output coupler (OC) in the test device was 0.06 dB. It can be seen that the PDL is very small, which is not enough to trigger Q-switching. Therefore, the saturable absorption characteristic of the SA plays a decisive role in our work.
3. Experimental setup
An erbium-doped fiber laser system based on the WTe2-graphene SA was experimentally constructed as shown in Fig. 4. The pump source was a laser diode (LD) with a wavelength of 976 nm, and the laser was delivered into the cavity by a 980/1550 nm wavelength division multiplexer (WDM). The length of the ring cavity was approximately 10 m, which was composed of single-mode fiber (7.6 m) and erbium-doped fiber (EDF, 2.4 m). The EDF was used as gain medium. A polarization-independent isolator (PI-ISO) was inserted to guarantee the unidirectional propagation of laser pulses. The polarization state in this cavity was adjusted by a polarization controller (PC), which could optimize the performance of Q-switched laser pulses. The forward laser pulse propagated into a coupler with 90% of the laser power input to the other end of the WDM. Another 10% of the laser was transmitted into another coupler to monitor the performance of the EDFL. The output optical spectrum was measured by an optical spectrum analyzer (OSA) with a resolution of 0.01 nm. Moreover, output pulse trains and frequency spectra were recorded by an oscilloscope and a radio-frequency (RF) spectrum analyzer, respectively.
4. Experimental results and discussion
The Q-switching technology based on SAs utilizes the nonlinear absorption characteristics of saturable absorbing media to change the absorption loss in the cavity to achieve Q-switching. In this experiment, using the saturable absorption characteristics of WTe2-graphene near the wavelength of 1550 nm, stable Q-switched pulses of the EDFL under different pump powers are obtained by changing the pump power and rotating the PC. When there is no WTe2-graphene SA in the EDFL, a continuous wave is generated from 15 mW by gradually increasing the pump power and rotating PC, while no laser pulse can be produced at any pump power. After integrating the SA into the ring cavity, it is not until the pump power reached 25.8 mW that the Q-switched pulses are observed. This is because, when the pump power is low, the SA has a large absorption coefficient, resulting in a high cavity loss. With the increase of the pump power, the number of the population inversion increases gradually, and the absorption coefficient of the SA decreases. When the absorption of the SA reaches saturation, the cavity loss is minimized and the Q value increases sharply, followed by the generation of Q-switched pulses [59]. The frequency spectra of Q-switched pulses under the pump power of 130.5 mW, 164.5 mW, 197.6 mW and 230.9 mW are plotted in Fig. 5(a). It is obvious that the intensity of these pulses is enhanced and the pulse interval becomes narrow with increasing the pump power. These results show that with the increase of the pump power, the power density of Q-switched pulses increases and the period of Q-switched pulses decreases. In the meantime, the number of Q-switched pulses increases and the pulse width decreases respectively. Figure 5(b) shows the stability of the optical spectra for Q-switched pulses under the pump power of 230.9 mW for 80 minutes. It is seen that output pulses with 3 dB bandwidth of 4 nm are obtained at the central wavelength of 1558 nm. Figure 5(c) exhibits the RF spectrum at the repetition frequency of 49.52 kHz under the same pump power. The signal-to-noise ratio (SNR) of 49.06 dB indicates the high stability of laser pulses.
Fig. 5. Pulse properties of the Q-switched EDFL with the WTe2-graphene SA. (a) Q-switched pulses under different pump powers; (b) Optical spectra of stable pulses; (c) RF spectra.
Moreover, we observe the variation trend of different parameters by changing the pump power. When the pump power increases from 25.8 mW to 279.9 mW, the repetition rate grows up from 13.3 kHz to 56.8 kHz, and the pulse width decreases from 5.5 µs to 1.2 µs, as shown in Fig. 6(a). The increasing pump power results in more gain provided to saturate the SA and the pulse is generated more rapidly, which makes the pulse duration decrease and the repetition rate increase [60]. The dependences of the pulse peak power and pulse energy on the pump power are depicted in Fig. 6(b) respectively. As the pump power rises from 25.8 mW to 279.9 mW, the pulse peak power grows linearly from 0.0989 mW to 3.1495 mW, meanwhile the pulse energy varies linearly from 0.44 nJ to 3.77 nJ.
Fig. 6. The pulse properties of the Q-switched EDFL with the WTe2-graphene SA. (a) The repetition rate and pulse-width versus pump power; (b) Peak power and pulse enenrgy versus pump power.
Table 1 presents the performance comparison of the passively Q-switched fiber lasers using the graphene SA, the WTe2 SA and the WTe2-graphene SA respectively. It is seen that the pump threshold power of the Q-switched fiber laser using the WTe2-graphene SA is very low, which is one order of magnitude smaller than that using the WTe2 only. In the meantime, the minimum pulse width of our laser is narrower than Q-switched pulses based on the graphene SA. Generally, a narrower pulse width can be obtained by increasing the pump power, but considering that overloading the pump power may have an irreversible impact on the stability of the SA, we do not increase the pump power further. The experimental results show that under the action of the WTe2-graphene composite SA, the Q-switched pulse output with low threshold and narrow pulse width is simultaneously achieved in the fiber laser, which confirms that such a SA is one of the most potential saturable absorbers at 1550 nm.
Table 1. Performance Comparison of Passively Q-switched Fiber Lasers Using the graphene SA, the WTe2 SA and WTe2-graphene SA respectively
5. Conclusion
In summary, the WTe2-graphene composite is deposited on a tapered fiber by photoinduced deposition method, which is used as a SA for a Q-switched fiber laser for the first time. The nonlinear absorption characteristics of the WTe2-graphene SA are measured by a balanced twin-detector measurement system. The saturation intensity, modulation depth and unsaturated loss are about 3.5817 MW/cm2, 3.78%, and 31.44%, respectively. Based on the WTe2-graphene composite SA, stable Q-switched pulses are obtained at the central wavelength of 1558 nm, and the threshold power is 25.8 mW. The repetition rate varies from 13.3 kHz to 56.8 kHz and the pulse width goes from 5.5 µs to 1.2 µs by increasing the pump power from 25.8 mW to 279.9 mW. Compared with the lasers based on SAs using only WTe2, the threshold pump power of the passive Q-switched fiber laser with the WTe2-graphene SA is reduced by one order of magnitude. In the meantime, the output pulse produced by the Q-switched fiber laser using the WTe2-graphene SA has narrower pulse width than that using graphene-only SA, which also has high repetition rate (56.8 kHz) and high SNR (49.52 dB). These results show that the WTe2-graphene composite can generate Q-switched pulses with low threshold power and narrow pulse width, and has broad application prospects and potential application value in ultrafast fiber lasers.
Funding
National Key Research and Development Program of China (2018YFE0117400); National Natural Science Foundation of China (NSFC) (61775074); China Postdoctoral Science Foundation (2018M642823); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20210324142010030).
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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