1. Introduction

Two-dimensional (2D) layered materials, owing to their unique electronic and optical properties, provide new opportunities for next-generation photonics technology [1–3 ]. Graphene, as a representative 2D material, has received much attention in recent years because it could find wide applications in various fields [4–6 ]. Benefiting from the distinguished advantages such as broadband saturable absorption and ultrafast optical response, graphene has been regarded as a promising nonlinear optical material which stimulates a variety of novel optical applications for photonics community. So far, various graphene-based photonic devices were fabricated, such as photodetectors [7], optical modulator [8,9 ] and saturable absorber (SA) [10,11 ]. In particular, the unique saturable absorption ability of graphene has aroused strong attention in the researches of ultrafast lasers [12–14 ]. Indeed, due to its excellent performance, graphene SA has been widely employed in various types of lasers to generate Q-switched or mode-locked pulses [15–20 ]. In addition to the saturable absorption effect, graphene was also discovered to possess large nonlinear refractive index [21,22 ], indicating that graphene could be used as a highly nonlinear medium for applications in the field of nonlinear optics [23–25 ]. Especially, the graphene-deposited microfiber photonic device (GMPD) has small size and possesses much higher nonlinear effect due to the large extension of interaction length between the graphene and the propagation light. Therefore, it is expected that the GMPD would find important applications where the high nonlinear effect is needed [26].

On the other hand, ultrafast fiber lasers, operating through the passive mode-locking, have attracted much attention due to the wide range of applications such as material processing, sensing and optical communications. In recent years, the improvements of ultrafast fiber laser performance have been paid more and more attention due to their great importance for the practical applications. As a key parameter of ultrafast laser, the pulse repetition rate is generally determined by the cavity length, suggesting that the repetition rate could be increased by reducing the cavity length. However, due to the limitation of the physical lengths of basic devices in the fiber laser cavity, the repetition rate could only reach to 10-20 GHz [27,28 ]. The high order harmonic mode-locking seems to be another solution to increase the repetition rate of mode-locked pulse. Nevertheless, the achieved maximum repetition rate is still limited within the order of GHz [29–31 ]. In fact, recently the dissipative four-wave-mixing (DFWM) mode-locking technique has been proposed to achieve ultrahigh repetition rate, i.e., hundreds of GHz. In order to achieve the DFWM mode-locking operation, a comb filter is always incorporated into the laser cavity. Beyond that a long segment of highly nonlinear fiber or high pump power is also needed [32–35 ]. As an alternative solution, the microring resonator, which simultaneously possesses highly-nonlinear and comb filtering effects, makes it possible to realize hundreds GHz mode-locked pulses in fiber lasers by DFWM mode-locking [36,37 ]. Nonetheless, the complicated manufacture of microring resonator, along with the larger insertion loss, limits its use in wide applications. Therefore, exploring other compact and fiber compatible photonic devices for DFWM mode-locking fiber lasers would be still meaningful for the ultrafast laser community. As mentioned above, the GMPD possesses highly nonlinear effect which is independent of operation wavelength [21,24 ]. Thus, a question naturally arises as to whether the GMPD could be employed as a highly nonlinear component to achieve ultrahigh-repetition rate pulse by DFWM mode-locking technique in fiber lasers.

In this work, we propose and demonstrate a 100 GHz high-repetition rate ultrafast fiber laser based on a GMPD. A Fabry-Perot (F-P) filter with a free spectral range of 0.8 nm is used to help achieve ultrahigh-repetition rate mode-locking pulse through the DFWM effect induced by the GMPD. The laser could operate stably in the case of simultaneously emitting as many as 18 lasing lines, corresponding to the repetition rate of 100 GHz. Further filtering experiments confirmed the quality of DFWM-induced mode-locking operation. The achieved results suggest that the GMPD could indeed function as a promising high-performance highly nonlinear component for delivering ultrahigh-repetition rate pulses from fiber lasers.

2. GMPD fabrication

In the experiment, the standard single-mode fiber (SMF) was stretched into a microfiber with a waist diameter of ~11 µm by utilizing the flame brushing technique. Injecting the visible light into the microfiber, the scattering evanescent field of the microfiber could be observed, as shown in Fig. 1(a) . Then the optical deposition method was employed to trap the graphene onto the microfiber. The details of deposition process have been described in [31]. Briefly, the prepared microfiber was immersed into the graphene/Dimethylformamide (DMF) solution with concentration of 0.07 mg/ml. Then the ASE light was injected into the microfiber. The optical force induced by the evanescent field of the microfiber was imposed on the graphene nanosheets for deposition. The process of deposition was in situ observed through the microscope with magnification of 100-fold. When the graphene deposition amount was enough, we turned off the light source. Here, the deposition length of graphene is 112 μm. The microscopy image of the fabricated GMPD is presented in Fig. 1(b), from which we can see that the graphene was well coated around the microfiber.

 

Fig. 1 (a) Scattering evanescent field of microfiber observed by the visible light; (b) Microscopy image of the fabricated GMPD.

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3. Laser performance and discussions

For further investigating the optical performance, the prepared GMPD was inserted into an erbium-doped fiber (EDF) laser. Figure 2 shows the schematic of the proposed fiber laser. A piece of 11 m EDF was used as the gain medium. Two polarization controllers (PCs) were employed to adjust the polarization state of the propagation light. Unidirectional operation of the fiber laser was ensured by a polarization-independent isolator (PI-ISO). A fiber pigtailed F-P filter with spectral spacing of 0.8 nm was employed to select the multiple lasing lines. The fineness and the peak-to-notch contrast ratio of the filter are 13 and 23.4 dB, respectively. The fundamental repetition rate is 6.8 MHz. Through a 10% port of an optical coupler, the laser output was monitored by an optical spectrum analyzer (Yokogawa AQ6317C), an oscilloscope (Tektronix DSA70804) with a 12.5 GHz photodetector (Newport 818-BB-35F) and a commercial autocorrelator (FR-103XL), respectively.

 

Fig. 2 Schematic of the proposed high-repetition rate fiber laser with GMPD.

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In the experiment, benefiting from the DFWM mode-locking operation induced by the GMPD, stable multiple lasing lines could be observed at a pump power of 22 mW. However, in this case the fiber laser only emitted 2 lasing lines. It is well known that the multiple DFWM processes would occur as the pump power level is high enough. Therefore, for better performance, we increased the pump power to 247 mW and fixed it in the following. Figure 3 shows the DFWM mode-locking operation at the pump power of 247 mW. As can be seen in Fig. 3(a), the spacing between the lasing lines is 0.8 nm, which corresponds to the spectral spacing of the F-P comb filter. It is worth noting that the intensity of the entire spectral lines decreases from center to edges significantly, which is the typical characteristic of DFWM mode-locking. Figure 3(b) illustrates the corresponding autocorrelation trace. The period of the pulse train is 10 ps, demonstrating that the laser delivers a pulse train of 100 GHz repetition rate. The repetition rate of the proposed DFWM mode-locked fiber laser is well consistent with the spacing of the lasing lines. If a Gaussian profile is assumed for fitting, the duration of the individual pulse in Fig. 3(b) is estimated to be 1.63 ps. Note that there presents subpulses in the autocorrelation trace of Fig. 3(b), which indicates that our laser output exhibits supermode-noise [38]. In order to reduce the supermode-noise, one could shorten the length of the laser cavity to increase the mode spacing, or adopt high fineness comb filter to reduce the number of oscillating modes in one peak of the filter.

 

Fig. 3 DFWM mode-locked operation: (a) Spectrum; (b) Autocorrelation trace.

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To verify whether the ultrahigh-repetition rate mode-locked operation was stable, we repeatedly scanned the laser output at 5-min interval in 75 mins. Figure 4 shows the measured results, in which we can see that no evident intensity fluctuations and wavelength drifts were observed. For better clarity, the power variation and wavelength drift of individual channels located at 1559.74 nm and 1560.54 nm were analyzed. From the measured data, the corresponding maximum power fluctuation is less than 0.36 dB and 0.76 dB, respectively. And the maximum wavelength drift of the both wavelengths is 0.01 nm. The stable operation indicated that the proposed fiber laser was well mode-locked by the DFWM effect.

 

Fig. 4 Repeatedly scanned output 15 times with a 5-min interval.

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In order to better investigate the characteristics of DFWM-induced mode-locking operation, an optical tunable filter (Santec OTF-350) was employed outside the laser cavity to filter out the wavelengths and measured their characteristics. The filtering results were presented in Fig. 5 . As can be seen in Figs. 5(a) and 5(b), when the multiple lasing lines was filtered to only one peak, no signal could be found on the autocorrelation trace, indicating that the single wavelength is not in mode-locking state. Then the bandwidth of the bandpass tunable filter was increased. As a result, the number of the filtered lasing lines was correspondingly increased. Figures 5(c)-5(h) show the case of the number of filtered lasing lines from 2 to 4. Here, when we filtered out 2 wavelengths, peaks with a period of 10 ps were equidistantly shown on the corresponding autocorrelation trace, indicating the 100 GHz pulse-train was caused by DFWM-induced phase-locked 2 lasing wavelengths. Then, gradually increasing number of the filtered laser lines from 2 to 4 shown in Figs. 5(c), 5(e), and 5(g), the corresponding autocorrelation traces illustrate that the pulses possess a same repetition rate but a narrower duration. The results demonstrate that a broader spectral envelope corresponds to a narrower pulse duration, which is similar to that of conventional mode-locking theory.

 

Fig. 5 (a), (c), (e), (g) Filtering spectra; (b), (d), (f), (h) Corresponding autocorrelation traces.

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In principle, DFWM is a parametric process among two pump photons with a signal and idler photon, which results in the emergence of phase-coherent signal and idler sideband at the expense of the pump. With proper conditions, the signal/idler sidebands could further successively stimulate higher orders sidebands. That is to say, the DFWM would intrinsically lead to the generation of phase-coherent sidebands with equal spacing. As all the sidebands keep a regular phase relationship, DFWM could induce passive mode-locking with high-repetition rate which was defined by the spectral spacing of lasing lines. In the laser cavity, if a comb filter, i.e. F-P filter, is applied, it helps to easily realize DFWM effect by combining a highly nonlinear photonic device. Although the imperfect filtering effect of the F-P filter would lead to generation of the weak “parasitic” pulse train at the fundamental repetition rate. However, it can be improved by adopting a comb filter with higher fineness. In our experiments, the proposed highly nonlinear photonic device was fabricated by depositing graphene onto a microfiber, which has a very compact design. In fact, the GMPD possesses two advantages: one is that the microfiber has a small diameter to exhibit a higher nonlinear coefficient, while the other is that the graphene itself has a highly nonlinear refractive index. Note that the accurate nonlinear coefficient could not be provided since the deposited amount of graphene could not be exactly measured. However, it is still expected that the nonlinear effect of the GMPD was greatly increased by extending the interaction length between the graphene and the propagation light. Therefore, the GMPD could effectively enhance the DFWM efficiency and sharply decrease the pump power to make it possible to achieve DFWM mode-locking at a low pump level. To be more scrupulously, we removed the GMPD from the fiber ring laser. In this case, even if the PCs or the pump power was adjusted in a large range, no DFWM mode-locked pulse was observed. The comparative results demonstrated that the GMPD indeed contributed to the DFWM mode-locking operation in our fiber laser.

4. Conclusion

In summary, we have demonstrated a 100 GHz ultrahigh-repetition rate mode-locked fiber laser operating in the communication band. The GMPD was proposed to generate the stable ultrahigh-repetition rate mode-locking operation by the DFWM effect. Further filtering experiments confirmed the quality of DFWM mode-locking. The experimental results demonstrated that the compact GMPD could serve as a promising high performance photonic device to realize ultrahigh-repetition rate mode-locking in fiber lasers or find other related applications in the field of nonlinear optics.