1. Introduction

Passively mode-locked Yb-doped fiber lasers (YDFLs) delivering ultrashort pulses have attracted much attention in recent decades due to their wide applications ranging from industrial to scientific research such as material processing, medicine, nonlinear microscopy, biology and nuclear physics [1,2]. Being different from the Er-doped fiber lasers, the YDFLs operate in the all-normal dispersion regime. Since the pulse evolution is strongly affected by the cavity dispersion, so far different pulse formation mechanisms and dynamics have been observed in passively mode-locked YDFLs. Generally, the passively mode-locked pulse evolved in YDFLs was believed to be conventional sech- or Gaussian-like dissipative soliton. However, recent achievements show that other solitons with different formation mechanisms could be also observed in YDFLs, such as similariton [3,4], bound solitons [5]. Very recently, the generation of rectangular pulse in fiber lasers has attracted much attention due to pulse energy scaling ability [6–10]. Indeed, the rectangular pulse could be experimentally obtained despite of dispersion regime and gain medium [11–15], even in all-normal-dispersion YDFLs [15]. Nevertheless, the aforementioned generation of rectangular pulse is only single wavelength operation. In some applications, fiber lasers that generate dual- or multi-wavelength pulses would be more favorable [16–21].

On the other hand, graphene has been demonstrated as an excellent nanomaterial for fabrication of saturable absorber (SA) [22–24]. The graphene saturable absorber (GSA) possesses several advantages such as wavelength-independent saturable absorption characteristic, low saturable absorption threshold, and large modulation depth. As for fabrication of fiber-compatible GSA, several methods have been proposed, such as using chemical vapor deposition [25,26], polymer nanocomposite [27,28], and optical deposition [29,30]. Based on the fabricated GSA, the passively mode-locked fiber laser generally delivered the conventional pulse with sech or Gaussian profile [22–30]. Recently, the microfiber was also proposed to deposit the graphene to form a SA [31]. By virtue of the geometric characteristic, the microfiber-based graphene was believed as a good candidate of nonlinear photonic device for investigating some nonlinear phenomena such as pulse shaping and four-wave-mixing [32,33]. Given that the multiwavelength mode locking and rectangular pulses are of great significance in field of laser physics, it would be interesting to develop a dual-wavelength rectangular pulse YDFL by using a microfiber-based GSA.

In this work, we demonstrated a dual-wavelength rectangular pulse YDFL by using a microfiber-based GSA. With the increasing pump power, the duration of dual-wavelength rectangular pulse varied from 1.41 ns to 4.23 ns. It was found that each wavelength corresponds to an individual nanosecond rectangular pulse by employing a tunable bandpass filter, where the dual-wavelength rectangular pulses show similar characteristics. In addition, the dual-wavelength switchable operation was also achieved. The demonstrated dual-wavelength rectangular pulse YDFL possesses flexible pulse output, which might open some new applications for the related fields.

2. GSA fabrication

In the experiment, the optical deposition method [34] was employed to fabricate the microfiber-based GSA. Firstly, the graphene/Dimethylformamide (DMF) solution with a concentration of 0.075 mg/ml was ultrasonicated for 30 minutes to ensure the uniformity of the solution. Then the standard single-mode fiber (SMF) was stretched into a microfiber with the waist diameter of ~6 µm by utilizing the flame brushing technique. Finally, the graphene/DMF solution was dripped around the waist region of the microfiber. An amplified spontaneous emission (ASE) light source was injected into the microfiber for optical evanescent-field deposition. The process of deposition was in situ observed through the microscope. When the graphene was deposited on the microfiber at a proper amount, we turn off the ASE light source. Then the deposited graphene was evaporated at room temperature. The microscopy image of the as-prepared microfiber-based GSA was presented in Fig. 1, showing that the graphene was uniformly and compactly deposited on the microfiber.

 

Fig. 1 Microscopy image of fabricated microfiber-based GSA.

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3. Laser performance and discussions

After preparing the microfiber-based GSA, we incorporated it into a Yb-doped fiber ring laser. The experimental setup of the dual-wavelength YDFL with a microfiber-based GSA was shown in Fig. 2. It has a ring cavity configuration made of pure normal-dispersion fibers. A piece of 3 m Yb-doped fiber (YDF) was used as the gain medium, which was pumped by a 980 nm laser diode with maximum output power of 370 mW after passing a 980/1060 nm wavelength-division multiplexer (WDM). The other fibers were all standard SMFs with a length of ~112 m. Thus, the total cavity length was ~115 m, corresponding to the cavity roundtrip time of ~561.8 ns. A bandpass filter centered at 1064 nm with a bandwidth of 8 nm was employed to obtain stable mode-locking operation [35]. To ensure unidirectional light propagation, a polarization-independent isolator (PI-ISO) was employed. A 20/80 coupler was used to output the laser emission. The output optical spectrum of the proposed laser was monitored by an optical spectrum analyzer (OSA). The pulse characteristics were detected by an oscilloscope (LeCroy Wave Runner 104MXi, 1 GHz) together with a 12.5 GHz photodetector (Newport 818-BB-35F) and a radio-frequency (RF) spectral analyzer (Advantest R3131A).

 

Fig. 2 Schematic of dual-wavelength rectangular pulse YDFL with a microfiber-based GSA.

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As we know, squeezing the fiber in the PCs could generate the polarization-dependent loss. The combination of the intracavity birefringence and the polarization-dependent loss could induce a comb filtering effect which could be used to achieve multiwavelength operation [16]. Therefore, when the pump power was increased to about 100 mW, single- and dual-wavelength cw operation could be easily observed by properly rotating the PCs. Figure 3(a) provides an example of the spectrum of dual-wavelength cw operation. Since the GSA was incorporated into the laser cavity, it was expected that dual-wavelength passive mode-locking could be achieved by increasing the pump power. Thus, in order to obtain dual-wavelength mode-locked pulses, we kept the cavity parameters and just increased the pump power when the dual-wavelength cw operation was achieved. In this case, the dual-wavelength mode-locked pulses were obtained as the pump power was increased to 263 mW. Figure 3(b) presents the typical dual-wavelength mode-locked spectrum. As can be seen from Fig. 3(b), the dual-wavelength simultaneously oscillated at 1061.8 and 1068.8 nm, with a 3dB bandwidth of 4.50 nm and 2.16 nm, respectively. It should be noted that the center wavelengths of dual-wavelength mode-locking are almost the same as those of cw operation. Then the dual-wavelength mode-locking in the time domain was investigated. Notably, the pulse profile exhibits rectangular shape on the oscilloscope trace, as shown in Fig. 3(c). Here, the pulse duration is 2.64 ns, as presented in the inset of Fig. 3(c). The pulse repetition rate is 1.78 MHz, which is determined by the cavity length. In the experiment, we also employed a commercial autocorrelator (FR-103XL) to check the fine structure of the rectangular pulse. However, no pulse signal could be detected with the autocorrelator, it is because the nanosecond duration of the rectangular pulse is beyond the maximum measurement range of the autocorrelator (90 ps), which indicates that the rectangular pulse is not the tight bunching of multi-soliton [36]. The RF spectrum presented in Fig. 3(d) revealed a fundamental cavity repetition rate of 1.78 MHz. The RF signal-to-noise ratio (SNR) over 50 dB confirmed that the mode-locking pulse was stable.

 

Fig. 3 Dual-wavelength operation. (a) cw laser operation; (b) Mode-locked spectrum; (c) Pulse-train. Inset: single pulse; (d) Corresponding RF spectrum.

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Then the pulse evolution with the variation of the pump power was investigated. Although dual-wavelength mode-locking state was initiated at the pump power of 263 mW in our experiment, the fiber laser could sustain the dual-wavelength mode-locking state when the pump power was decreased to ~180 mW due to the pump hysteresis phenomenon [37]. Figure 4(a) shows the pulse width broadening when the pump power was increased. As can be seen in Fig. 4(a), the pulse duration varied from 1.41 ns to 4.23 ns as the pump power was increased from 187 mW to 370 mW. In addition, the pulse amplitude was almost clamped at a constant level during the pulse broadening. It should be also noted that during the process of the rectangular pulse broadening, the 3-dB spectral bandwidths of the dual-wavelength mode-locked spectra were almost invariable despite of the slightly increasing spectral intensity. These rectangular pulse characteristics mentioned above are the same as those in dissipative soliton resonance (DSR) region [11–15]. Therefore, the dual-wavelength mode-locked rectangular pulse could be regarded as DSR pulse. Figure 4(b) further shows the experimentally measured pulse duration and the average output power versus the pump power. Here, the pulse width increased monotonically to 4.23 ns when the pump power was up to 370 mW. Correspondingly, the maximum output power of rectangular pulse is 3.05 mW under the pump power of 370 mW. Since the pulse repetition rate is 1.78 MHz, the maximum output pulse energy was 1.713 nJ.

 

Fig. 4 (a) Dynamics of pulse broadening with the pump power increased; (b) Experimentally measured pulse width and average output power versus the pump power.

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To further study the characteristics of the dual-wavelength mode-locking operation, a bandpass filter with a tuning range from 1020 nm to 1090 nm was employed to resolve the lasing at each wavelength. The filtered spectra are shown in Fig. 5. Here, the initial dual-wavelength spectrum (black dotted curve) can be resolved as two independent spectra (blue and red curves), corresponding to the two spectral humps centered at 1061.8 nm and 1068.8 nm, respectively. The filtered pulse-trains centered at two wavelengths are shown in Figs. 6(a) and 6(c), suggesting that both of the two wavelengths operated individually in passive mode-locking state. For better clarity, the oscilloscope traces measured with small scan range are also shown in the insets of Figs. 6(a) and 6(c). The pulse profiles of both the two wavelengths are rectangular. The duration of the pulses were slightly different with each other, which were measured to be 2.41 ns (1061.8 nm) and 2.20 ns (1068.8 nm), respectively. In addition, we have also measured the RF spectra of the mode-locked pulse-trains at two wavelengths. The SNR of the two wavelengths are ~50 dB, as presented in Figs. 6(b) and 6(d).

 

Fig. 5 Filtered spectra of dual-wavelength rectangular pulse operation.

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Fig. 6 Wavelength resolved operation. (a) and (c) The pulse-trains (Inset: single pulse) centered at 1061.8 nm and 1068.8 nm, respectively; (b) and (d) Corresponding RF spectra.

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With the further orientations of the PCs, the cavity loss for the dual-wavelength pulses could vary, which could be employed for the wavelength switchable operation of dual-wavelength rectangular pulses [16]. Figure 7 shows the wavelength switchable operation of the proposed rectangular pulse fiber laser by carefully rotating the PCs. As can be seen in Figs. 7(a) and 7(c), the center wavelengths of the switchable rectangular pulses are 1061.8 nm and 1068.8 nm, respectively, in agreement with the central wavelengths of the dual-wavelength rectangular pulse operation. The corresponding pulse-trains are shown in Figs. 7(b) and 7(d), whose pulse repetition rates are both ~1.78 MHz. In addition, the pulse profiles centered at two wavelengths were also shown in the insets of in Figs. 7(b) and 7(d), which are both rectangular shapes with 2.56 ns and 2.17 ns durations, respectively. In the case of wavelength switching operation, the output pulse was still checked by a commercial autocorrelator. However, the same as the case of dual-wavelength rectangular pulse operation, no autocorrelation trace could be detected again.

 

Fig. 7 Wavelength switchable operation of the proposed rectangular pulse fiber laser: (a) and (c) The spectra centered at 1061.8 nm and 1068.8 nm, respectively; (b) and (d) Corresponding pulse-trains (Inset: single pulse).

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In our experiment, we noted that the maximum pulse energy under the available pump power level is only about 1.713 nJ, which might be a little low. It could be mainly because the insertion loss of the microfiber-based GSA is not small enough, i.e., ~6.7 dB in this experiment. Thus, the output pulse energy could be improved by further fabricating a higher performance GSA with low insertion loss. In addition, only the dual-wavelength mode-locking operation was obtained, which could be due to the large channel spacing of the intra-cavity birefringence-induced comb filter. Therefore, in order to obtain multiwavelength mode-locked rectangular pulses, we could employ a comb filter with narrower channel spacing in the fiber laser, such as high-birefringence-based comb filter [38]. Although the generation of the rectangular pulse was only reported in this work, in the current laser system we have also observed the tightly bunched solitons. It was also found that the tightly bunched solitons could result in the ‘rectangular’ shape pulse shown on the oscilloscope trace due to the limited bandwidth of the used oscilloscope [36]. However, different from the rectangular pulse reported in this work, when achieving the tightly bunched solitons, the autocorrelation trace could be clearly observed. Based on the above experimental results, we could infer that the rectangular pulse obtained in this work is a single coherent pulse.

4. Conclusion

In conclusion, we have demonstrated the generation of dual-wavelength rectangular pulse from a YDFL with a microfiber-based GSA. After being resolved by a tunable bandpass spectrum filter, it is found that each wavelength corresponds to an independent rectangular pulse. The duration of dual-wavelength rectangular pulse broadened from 1.41 ns to 4.23 ns with the increasing pump power. Moreover, by properly rotating the PCs, the dual-wavelength switchable operation was also obtained. Such a dual-wavelength laser with flexible rectangular pulse outputs at 1.0 μm would find some applications in spectroscopy, biomedicine and sensing research.