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We reported on the generation of high-order harmonic mode-locking in a fiber laser using a microfiber-based molybdenum disulfide (MoS2) saturable absorber (SA). Taking advantage of both the saturable absorption and large third-order nonlinear susceptibilities of the few-layer MoS2, up to 2.5 GHz repetition rate HML pulse could be obtained at a pump power of 181 mW, corresponding to 369th harmonic of fundamental repetition frequency. The results provide the first demonstration of the simultaneous applications of both highly nonlinear and saturable absorption effects of the MoS2, indicating that the microfiber-based MoS2 photonic device could serve as high-performance SA and highly nonlinear optical component for application fields such as ultrafast nonlinear optics.
© 2014 Optical Society of America
1. Introduction
Passively mode-locked fiber lasers have been widely investigated due to their potential applications ranging from optical communications, optical sensing, material process, and medicine [1]. So far, there are several approaches to achieve passive mode locking in a fiber laser, such as nonlinear polarization rotation [2,3], nonlinear amplifying loop mirror [4,5], and real saturable absorber (SA) [6–13]. Different from the other two techniques, incorporating a real SA is deemed to be the most efficient way to obtain mode-locking pulse without accurately adjusting the polarization states. Among the various types of real SAs, the semiconductor saturable absorber mirrors (SESAMs) [6,7], single-wall carbon nanotubes (SWNTs) [8,9], and graphene [10–13] have been found to be effective laser mode lockers. In particular, by virtue of the advantages such as broadband saturable absorption, low saturable absorbing threshold and large modulation depth, the graphene, a type of two-dimensional nanomaterials, was believed as an excellent material for fabrication of high-performance SA.
Following the same vein on graphene investigations, the exploration of graphene analogue materials for fabrication of SA become a hot topic in ultrafast laser community. In recent years, Topological Insulators (TIs), as one of the graphene analogues, has been well studied as high-performance photonic device with broadband saturable absorption and highly nonlinear effect in fiber lasers [14–19]. Based on the TISA, both the passive mode-locking [14,15,17,19] and Q-switching [16,18] were achieved. Later, layered molybdenum disulfide (MoS2) was also found to be another graphene analogue. By using the open-aperture Z-scan technique, Wang et al. investigated the nonlinear optical properties of layered MoS2 [20]. The results demonstrated that the MoS2 exhibits stronger SA response at 800 nm than graphene. Then, by considering the presence of both 1T (metallic) and 2H (semiconducting) phases in the MoS2 nanoplatelets [21], Zhang et al. demonstrated the broadband saturable absorption at 400, 800, and 1060 nm wavebands [22]. Enlighten by the results reported in [22], recently we firstly demonstrated the achievement of femtosecond pulse from an erbium-doped fiber (EDF) laser operating at 1550 nm waveband by using filmy few-layer MoS2 SA [23], which further expands the saturable absorption waveband of few-layer MoS2. Then, a picosecond pulse EDF laser with a CVD-grown MoS2 SA was also achieved [24]. In fact, apart from the broadband saturable absorption, MoS2 was also found to possess large third-order nonlinear susceptibilities on the order of 10−19m2/V2 [25], which could be employed to investigate the nonlinear optical phenomena. Therefore, the MoS2 could be potentially used to fabricate photonic device with both the high optical nonlinearity and saturable absorption.
Regarding the mode-locked fiber lasers, the nonlinear effect inside the laser cavity could play an important role in the evolution and dynamics of mode-locked pulses. In particular, a direct application of introducing proper nonlinear effect is to achieve harmonic mode locking (HML) of a fiber laser, which could be used to obtain high repetition rate pulse with the orders of GHz [26–28]. As we know, high repetition rate pulse fiber lasers have attracted considerable attention in recent years due to the wide applications in fields such as astronomical frequency combs and fiber communication system. Moreover, by increasing the interaction length between the SA nanomaterial with highly nonlinear effect and propagation light, it has been demonstrated that the nanomaterial-deposited microfiber could be employed as highly nonlinear SA photonic device for nonlinear pulse shaping in fiber lasers [17]. Thus, taking into account the nonlinear optical properties of MoS2 and the wide applications of high-repetition rate fiber lasers, it would be interesting to know whether the microfiber-based MoS2 SA could be employed in fiber lasers to generate high-order HML pulses.
In this contribution, we demonstrated a high-order passively HML erbium-doped fiber (EDF) laser using a microfiber-based MoS2 SA. By simply increasing the pump power level, up to 2.5 GHz repetition rate HML pulse could be achieved in the fiber laser, which corresponds to 369th harmonic of fundamental repetition frequency. The experimental results demonstrated that the microfiber-based MoS2 SA could serve as a high-performance photonic device with both functions of high optical nonlinearity and saturable absorption, which could find important applications in fields such as ultrafast nonlinear optics.
2. Fabrication and characteristics of the microfiber-based MoS2 SA
The MoS2 nanosheets were synthesized using the convenient and cost-effective hydrothermal intercalation and exfoliation method [21,22]. The MoS2 nanosheets were dispersed in alcohol with a concentration of 0.018 mg/ml. The scanning electron microscopy (SEM) image of the MoS2 nanosheets is shown in Fig. 1(a), where we can clearly identify the layered structure. To confirm that the MoS2 has been exfoliated to be few-layer structure, Fig. 1(b) illustrates the Raman spectrum. Both the in-plane vibrational mode E12g at 383.5 cm−1 and the out-of-plane vibrational mode A1g at 407.7 cm−1 are clearly exhibited, in agreement with previous studies of MoS2 [29,30]. Note that the frequency of E12g peak decreases while that of the A1g peak increases with the increasing layer number. Therefore, the number of layers of the MoS2 could be estimated by the separation between the two modes [30]. Considering the ~24.2 cm−1 frequency difference, the MoS2 nanosheets used here is with a thickness of 4-5 layers.
The standard single-mode fiber (SMF) was stretched into a microfiber with the waist diameter of ~11 µm by utilizing the flame brushing technique [31]. The optical force to deposit the MoS2 onto the microfiber is induced by the evanescent field of the microfiber. The detailed nanomaterial deposition process by using evanescent field of a microfiber has been described in [32]. The deposition process was in situ observed through the microscope with a magnification of 100-folds. After evaporating at room temperature, the microfiber-based MoS2 SA was accomplished, as shown in Fig. 2(a). After the fabrication of the microfiber-based MoS2 SA, we measured the saturable absorption of the microfiber-based MoS2 SA utilizing the power-dependent transmission technique. With the same experimental setup with [17], the saturable absorption characteristic is shown in Fig. 2(b), in which we can see that the modulation depth is ~2.82% and the non-saturable loss is ~57.34%. Note that the modulation depth and non-saturable loss of the prepared SA are slightly inferior to the previously reported PVA-based MoS2 SA [23]. However, it could be further improved by optimizing the amount of deposited MoS2 and the diameter of microfiber. And it should be also noted that the saturable absorption curve of the as-prepared MoS2 SA has been measured for several times in our experiment and the curves similar to Fig. 2(b) were obtained. Moreover, we have also measured the polarization properties of the prepared MoS2 SA and no evident polarization-dependent loss of the fabricated MoS2 SA could be observed.
Fig. 2 (a) Microscopy image of microfiber-based MoS2 SA; (b) Measured saturable absorption curve and the corresponding fitting curve.
3. Laser performance and discussions
In order to check the laser performance based on the fabricated MoS2 SA, it was inserted into an EDF ring laser. The schematic of the proposed EDF laser with microfiber-based MoS2 SA was shown in Fig. 3. A ~5m EDF was used as the gain medium. To ensure unidirectional light propagation, a polarization-independent isolator (PI-ISO) was employed. Two polarization controllers (PCs) were employed to adjust the polarization state of the propagation light. Taken by a 10% fiber coupler, the laser output is monitored by an optical spectrum analyzer (OSA, Anritsu MS9710C), a 2 GHz oscilloscope (LeCroy WaveRunner 620Zi) with a 12.5 GHz high speed photodiode detector (New Focus P818-BB-35F). In addition, the corresponding pulse profile has been identified with an autocorrelator (FR-103XL).
In the experiment, the fiber laser always tended to operate in HML (multi-pulse) state due to the highly nonlinear effect induced by the microfiber-based MoS2 SA. However, taking advantage of the pump hysteresis phenomenon, we could still obtain the fundamental mode-locking state by carefully decreasing the pump power to 8.84 mW, which is shown in Fig. 4. As can be seen from Fig. 4(a), the output spectrum is centered at 1556.86 nm with a 3-dB bandwidth of 2.47 nm. The measured oscilloscope trace of the output pulse-train is shown in Fig. 4(b). The pulse repetition rate is calculated to be 6.77 MHz, which is well consistent with the cavity length 30.17 m. Because the output power was measured to be only 65 µW, the pulse duration could not be measured due to insufficient sensitivity of the autocorrelator.
When the pump power was gradually increased, we could obtain different orders of HML states. In our experiment, the highest HML pulse repetition rate we could measure was 2.5 GHz, corresponding to 369th harmonic of fundamental repetition frequency. This HML state was achieved at the pump power of 181 mW. Figure 5 presents the performance of the 2.5 GHz HML operation. The mode-locked spectrum with a 3-dB bandwidth of 0.94 nm was shown in Fig. 5(a). The recorded pulse train is presented in Fig. 5(b). The repetition rate is 2.5 GHz, corresponding to an interval between pulses of 0.4 ns. For the purpose of clarity, we also provide the pulse-train with larger span in the inset of Fig. 5(b). Figure 5(c) illustrates the corresponding autocorrelation trace, which is measured to be 3.0 ps if sech2 shape is assumed. Thus, the time-bandwidth product is 0.348, showing that the output pulse is slightly chirped.
Fig. 5 HML operation at the repetition rate of 2.5 GHz. (a) Mode-locked spectrum; (b) Recorded pulse-train. Inset: pulse-train with larger span; (c) Autocorrelation trace.
As we know, the typical characteristic of the passively HML in fiber lasers is that the harmonic number mainly depends on the pump power. It indicates that we could conveniently tune the repetition rate in the allowable range just by adjusting the pump power. Figure 6 illustrates the harmonic number and the output power with respect to the pump power. It is evident that both the harmonic number and the output power linearly scale with the pump power level. The highest harmonic number is 369, corresponding to repetition rate of 2.5 GHz. Here, it is worth noting that the pulse repetition rate still could be increased when the pump power was further increased. However, we only showed the results with ~2.5 GHz pulse repetition rate in this contribution because of the bandwidth limitation of the used oscilloscope. It should be also noted that the major limitation to further scaling in the repetition rate of the HML pulse could be the optical damage of the microfiber-based MoS2 SA. In addition, the output power varied from 2.51 mW to 5.39 mW when the pump power was adjusted from 91.5 to 181 mW. In the experiment, the role of the microfiber-based MoS2 SA in mode-locked operation was also verified. By removing the MoS2 SA from the fiber laser, the passively mode-locked pulse could not be observed even if the cavity parameters were adjusted in a large range. The comparative results demonstrated that the microfiber-based MoS2 SA was responsible for the HML operation of the proposed fiber laser.
4. Conclusion
In conclusion, we have demonstrated a high-order passively HML EDF laser with a microfiber-based MoS2 SA. By virtue of the saturable absorption and large third-order nonlinear susceptibilities of the few-layer MoS2, high repetition rate (up to 2.5 GHz) of HML pulse had been obtained, which corresponds to 369th harmonic of fundamental repetition frequency. These results suggest that the microfiber-based MoS2 SA could be employed as a high-performance photonic device with both the high optical nonlinearity and saturable absorption, which could find versatile applications in the related fields of photonics.
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 61378036, 61307058, 11304101, 11474108, 11074078), the PhD Start-up Fund of Natural Science Foundation of Guangdong Province, China (Grant No. S2013040016320), and the Scientific and Technological Innovation Project of Higher Education Institute, Guangdong, China (Grant No. 2013KJCX0051). Z.-C. Luo acknowledges the financial support from Zhujiang New-star Plan of Science & Technology in Guangzhou City (Grant No. 2014J2200008).
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