We reported on the dissipative soliton resonance (DSR) phenomenon in a mode-locked Yb-doped fiber laser by using the nonlinear polarization rotation technique. It was found that the multi-pulse oscillation under high pump power could be circumvented by properly adjusting the polarization controllers, namely, the wave-breaking-free rectangular pulse in DSR region was achieved. As the DSR signature, the pulse duration varied from 8.8 ps to 22.92 ns with the increasing pump power. Correspondingly, the maximum pulse energy was 3.24 nJ. The results demonstrated that the DSR phenomenon could exist in Yb-doped fiber lasers, which could be used to achieve wave-breaking-free, ultrahigh-energy pulse.
© 2013 Optical Society of America
Yb-doped fiber lasers operating in all-normal dispersion regime have attracted much attention in the past decade due to the high-energy pulse output, which possess various applications in industrial and scientific research such as material processing, medicine, biology, nonlinear microscopy and nuclear physics. Generally, the pulse energy of a mode-locked fiber laser operating in the anomalous-dispersion regime is limited to 0.1 nJ by the soliton area theorem . At higher energy, wave breaking occurs which is manifested as multiple pulsing due to the overdriven nonlinear effect . Being different from the case of anomalous-dispersion operation regime, a large chirp could be generated on the mode-locked pulse in a passively mode-locked Yb-doped fiber laser with normal dispersion regime, which reduces the nonlinear effects and thus allows high-energy pulse operation . With the development of theories and experiments in the fields of nonlinear optics and laser physics, various pulse shaping mechanisms have been investigated in passively mode-locked Yb-doped fiber lasers, i.e., dissipative solitons [4,5] and similaritons [6,7]. Among different pulse formations, the generation of similariton was believed as an outstanding way to scale pulse energy due to the high tolerance of cavity nonlinear effect . However, as the pump power increases, previous reports have shown that the wave breaking of similaritons would also occur [9,10]. In order to further reduce the cavity nonlinear effect, the microstructure large mode area (LMA) gain fiber was introduced to construct the fiber laser [11–13]. Nevertheless, the fusion splicing between the microstructure LMA and single mode fiber is difficult, which means that the bulk components should be employed and the fiber laser is a non-all-fiber one. From the viewpoint of practical applications, the all-fiber design is more favorable owing to the compact, misalignment-free, and environmentally stable characteristics. Therefore, there would be a strong motivation to develop an all-fiber wave-breaking-free, high energy pulse Yb-doped fiber laser.
To overcome the nonlinear effect on the pulse breaking phenomenon, an efficient way is to develop new pulse formation mechanisms. Recently, a novel type of pulse formation called dissipative soliton resonance (DSR) was theoretically proposed to achieve wave-breaking-free pulse by properly selecting parameters in the frame of complex Ginzburg–Landau equation [14–19]. It was shown that the pulse formed under the DSR condition could increase its energy and width indefinitely with the increasing pump power. Meanwhile, the pulse keeps rectangular shape with the constant amplitude. The experimental observations confirmed that the DSR phenomenon could indeed exist in fiber lasers . However, up to now, the DSR phenomenon was only observed in Er-doped fiber laser at 1.55 wavelength region [20–23]. According to the theoretical prediction, the DSR phenomenon is independent of the laser gain medium and operation wavelength [14,16]. Considering the excellent pumping efficiency of Yb-doped fiber which is suitable for the generation of high-energy pulse, the achievement of DSR pulse in Yb-doped fiber lasers at 1.0 wavelength region would be of great significance to the field of laser physics.
In this work, we reported on the observation of DSR phenomenon in a passively mode-locked Yb-doped fiber laser using nonlinear polarization rotation (NPR) technique. It was experimentally found that the multi-pulse (or wave-breaking) operation could be suppressed by properly rotating the polarization controllers (PCs). After the suppression of the multi-pulse oscillation, the rectangular pulse duration broadened from 8.8 ps to 22.92 ns with the increasing pump power while keeping their amplitude constant, indicating that the mode-locked pulse in DSR region was obtained. At a maximum pump power of 400 mW, the pulse energy could be up to 3.24 nJ. The experimental results provide the first observation of DSR phenomenon in Yb-doped fiber laser, which would be the powerful evidence for the generation of the ultrahigh energy, wave-breaking-free pulse at 1.0 wavelength region.
2. Experimental setup
The schematic of the Yb-doped fiber laser system is shown in Fig. 1. A piece of 4 m Yb-doped single mode fiber with an absorption coefficient of 31.5 dB/m at 975 nm wavelength is used as the gain medium, pumped by a 975 nm laser diode with the maximum pump power of 400 mW via a 980/1060 nm wavelength-division multiplexer (WDM). The other fibers in the laser cavity are 46 m HI-1060 fiber. Thus, the whole cavity length is about 50 m, corresponding to the cavity roundtrip time of 256 ns. Since the laser cavity is an all-normal-dispersion one, a fiber-pigtailed bandpass filter centered at 1060 nm with a bandwidth of 8 nm is inserted in the cavity to obtain stable mode-locking . The polarization-dependent isolator ensures the unidirectional operation and polarization selectivity. Two PCs were employed to adjust the polarization state of the circulating light. An optical coupler with 10% output serves as the output port. A 20:80 coupler is used for observing the laser spectrum and pulse train simultaneously. The laser spectrum is measured by an optical spectrum analyzer (Yokogawa AQ-6370C). The pulse train is detected by an oscilloscope (LeCroy WaveRunner 620Zi) with a high-speed photodetector (ET-3000AFC, EOT). The pulse duration is also monitored with a commercial autocorrelator (Femtochrome FR-103XL).
3. Experimental results and discussions
The proposed Yb-doped fiber laser was mode locked with NPR technique. As the PCs were properly rotated, the fiber laser achieved mode-locking state with the pump power of 190 mW. Generally, the conventional dissipative soliton with steep spectral edges operating in all-normal-dispersion regime was obtained. A typical spectrum of dissipative soliton at 200 mW pump power was shown in Fig. 2(a). The center wavelength and the 3-dB spectral bandwidth are 1067.05 nm and 3.31 nm, respectively. Figure 2(b) presents the corresponding pulse-train, which operates at the fundamental repetition rate of 3.9 MHz. The pulse-train was stable at this mode-locking state. However, with the PCs fixed, as we increased the pump power the multi-pulse oscillation occurred, indicating that the wave breaking could not be suppressed in this case. In the experiment, with the variation of the pump power, the pulse number within the laser cavity could be increased monotonically from 1 to 8. Typically, Fig. 2(c) shows the 5-pulse operation when the pump power is 300 mW. Here, the pulse repetition rate is 29.84 MHz. The observation of multi-pulse formation with the increasing pump power was well consistent with the previous reports [25–27].
As we know, in the NPR-based fiber ring laser, the pulse dynamics is strongly affected by the PC settings and pump power level. Therefore, varying these cavity parameters allows us to observe various types of pulse formations. When we further adjusted the PCs, it was found that a notable pulse behavior was shown on the oscilloscope. The notable pulse features that the pulse duration broadens with the increasing pump power while the peak of the pulse almost keeps constant and the pulse profile is rectangular. In addition, no pulse splitting was observed with the high pump power level, suggesting that the multi-pulse oscillation was completely suppressed with the appropriate cavity parameters. It should be noted that the aforementioned characteristics are the typical signatures of DSR phenomenon. In the following, we will show the pulse characteristics under the DSR condition. Generally, the self-starting rectangular pulse in DSR region could be obtained at the pump power of 200 mW. However, the mode-locked rectangular pulse can be sustained when the pump power was decreased to 175 mW due to the pump hysteresis phenomenon in fiber lasers [25–27]. The typical spectrum of mode-locked pulse in DSR region was shown in Fig. 3. As can be seen in Fig. 3, the central wavelength of the mode-locked spectrum is 1061.04 nm and the 3-dB bandwidth is 2.11 nm. Moreover, there is a spectral peak appeared on the top of mode-locked spectrum. The spectral peak was not so stable in the experimental observation, indicating that the spectral peak could be the continuous wave component on the mode-locked spectrum.
As mentioned above, the mode-locked pulse could be also observed at 175 mW pump power due to the pump hysteresis phenomenon. Figure 4(a) presents the mode-locked pulse-train at 175 mW pump power. In this case, the pulse profile is Gaussian-like but not rectangular. Then an autocorrelator was employed to measure the pulse duration. As shown in the inset of Fig. 4(a) with green curve, the pulse duration is 8.8 ps if a Gaussian pulse profile is assumed. Therefore, the time-bandwidth product is 4.95, showing that the pulse is chirped. When the pump power was slightly increased, the pulse duration exhibits the broadening trend, as shown in the inset of Fig. 4(a) with blue curve. By further increasing pump power, the pulse evolved into rectangular shape and the pulse duration broadened obviously. Figure 4(b) shows the pulse-train at the pump power of 300 mW, where the rectangular pulse profile was clearly shown on the oscilloscope. Here, no pulse could be detected with the autocorrelator in this case due to the large pulse duration. The observed pulse profile evolution from Gaussian-like to rectangular is in agreement with the theoretical prediction  and previously experimental demonstration . It should be noted that no fine structure of the autocorrelation trace or pulse bunching was observed with the increasing pump power, showing that the broadening pulse is single-pulse operation and the pulse operates in DSR region.
In order to investigate the pulse evolution in more detail, Fig. 5(a) shows the dynamic of pulse broadening as a function of the pump power. Obviously, it evolves from a Gaussian-like pulse into a rectangular one as the pump power is increased. Correspondingly, the duration of the mode-locked pulse broadens gradually with the pump power increasing from 175 mW to 400 mW, while the peak of the pulse remains constant at high pump power. It is worth noting that the 3-dB bandwidth of the mode-locked spectrum is almost constant at ~2 nm as the pump power varies despite of the increasing intensity. For the purpose of better studying the pulse characteristics, Fig. 5(b) further shows the experimentally measured pulse width and average output power versus the pump power level. As can be seen from Fig. 5(b), the pulse width increased monotonically to 22.92 ns when the pump power changes from 175 mW to 400 mW. Correspondingly, the highest laser output power is 12.65 mW under the pump power of 400 mW. Considering the cavity repetition rate of 3.9 MHz, the measured largest output pulse energy was 3.24 nJ, which was limited by the pump power level.
In our experiment, although the multi-pulse operation was observed, the rectangular pulse without wave-breaking could be always attained by properly rotating the PCs. In NPR-based fiber ring laser, the adjustments of PCs affect many factors such as the cavity loss, spectral filtering, cavity feedback effects and cavity dispersion, which are related to the generation of DSR pulse in mode-locked fiber lasers [14,18]. Thus, with fine tuning of the PCs the wave-breaking-free pulse in DSR region could be obtained, which was similar with those in Er-doped fiber lasers. Moreover, it should be noted that the highest pulse energy obtained in the proposed fiber laser is 3.24 nJ. The achievable pulse energy is dependent of the pump power level and the doping concentration of Yb fiber. Therefore, the pulse energy could be further increased if the higher output power of pump laser source and highly doped Yb-fiber were employed. Finally, the NPR technique was employed to achieve the mode-locking operation, which was an artificial saturable absorber (SA) effect generated by the fiber nonlinear effect. Thus, the damage threshold of NPR-based SA is much higher than other SAs (such as semiconductor saturable absorption mirror, graphene, carbon nanotube), which is much more suitable for the generation of high energy pulse in fiber lasers. As a consequence, the demonstration of DSR phenomenon in Yb-doped fiber laser based on NPR technique is of great significance in the field of high-energy pulse lasers.
In summary, we have demonstrated the existence of DSR phenomenon in a passively mode-locked Yb-doped fiber laser with the NPR technique. By properly adjusting the PCs, the wave-breaking-free pulse operating in DSR region was obtained. Therefore, the multi-pulse oscillation under high pump power which generally occurs in passively mode-locked fiber laser could be effectively suppressed in this case. Under the DSR condition, the output pulse duration changed from 8.8 ps to 22.92 ns with the increasing pump power, corresponding to a maximum pulse energy of 3.24 nJ. The results demonstrated that the DSR phenomenon could be observed in Yb-doped fiber laser, which would be beneficial to achieve the ultrahigh energy, wave-breaking-free pulse at 1.0 wavelength region.
This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11074078, 61378036, 61307058, 11304101), the Specialized Research Fund for the Doctoral Program of Higher Education, China (Grant No. 20094407110002), the Key Program for Scientific and Technological Innovations of Higher Education Institutes in Guangdong Province, China (Grant No. cxzd1011), and the Natural Science Foundation of Guangdong Province, China (Grant No. S2013040016320).
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