We demonstrated dissipative soliton obtained from a graphene oxide mode-locked Er-doped fiber laser, which operated in normal dispersion cavity by employing the dispersion compensation fiber. The highly chirped pulses at the repetition rate of 19.5 MHz can be compressed from 11 ps to 542 fs by using single mode fiber. Numerical simulations were in good agreement with the experimental results. The hydrophilic graphene oxide with easier fabrication shows great potential to be a novel low-cost saturable absorber in reliable and compact mode-locked fiber laser system.
©2012 Optical Society of America
Since the first demonstration on graphene mode-locked fiber lasers in 2009, graphene has been widely investigated as a novel saturable absorber [1–15], due to its strong saturable absorption and broad operational wavelength range. And pulses from femtosecond to picosecond level have been demonstrated in graphene mode-locked Er-doped fiber lasers operated from anomalous to normal dispersion regimes. Conventionally, the majority of reports focused on anomalous dispersion cavity, and the shortest pulse so far was 174 fs achieved from a near-zero dispersion cavity . In normal dispersion regime, the dissipative soliton with minimum pulse width of 40 ps was generated from an Er-doped fiber laser with graphene based saturable absorber which was fabricated by chemical reduction method .
More recently, the researchers have started to pay much attention to the graphene oxide material and its application on mode-locked fiber lasers [6, 16–18] as well as on crystal-based solid-state lasers [19, 20]. Graphene oxide is an atomically thin sheet of carbon bonded with oxygen functional groups. It can be produced by the oxidative treatment of graphite , which is the first step of graphene fabrication by chemical reduction method . Therefore, as a semi-product, the graphene oxide is easier and faster to obtain than graphene. Moreover, the covalent oxygen functional groups in graphene oxide not only render strong hydrophilic property, but also give rise to remarkable mechanical strength [23, 24], which offers superior flexibility and processibility for production of graphene oxide based optoelectronics. Further investigation shows graphene oxide has ultrafast recovery time and strong saturable absorption, which is comparable to that of graphene [25, 26]. In , Sobon et al. compared the mode-locking performance between graphene oxide (GO) and reduced graphene oxide (rGO) in the same cavity configuration and both of them generated 390 fs pulses with single pulse energy of ~30 pJ. In , we reported the graphene oxide mode-locked fiber lasers operated in large anomalous dispersion cavity and near zero dispersion cavity respectively: (1) in anomalous dispersion cavity, the laser generated conventional soliton with 600 fs pulse width and the pulse energy was 130 pJ; (2) in near zero dispersion cavity, 200 fs stretched pulses with 250 pJ single pulse energy was obtained with dispersion management. However, to the best of our knowledge, the graphene oxide mode-locked dissipative soliton fiber laser has never been demonstrated before.
It is well known that the single pulse energy of passively mode-locked Er-doped fiber lasers with anomalous dispersion was restricted to picojoule-level by the soliton area theory. An attractive way to increase the pulse energy in mode-locked fiber oscillator is to shift the mode-locking operation into the normal dispersion regime, where the dissipative soliton with large normal chirp can be generated . Due to its strongly chirped feature, the pulse can be easily amplified. Additionally, the linear chirp can be efficiently removed by employing a segment of single mode fiber, allowing high-quality pulse compression in an all-fiber configuration.
Here, we reported a graphene oxide mode-locked dissipative soliton fiber laser with extra-cavity pulse compression. The laser generated dissipative soliton with positive chirp and steep spectral edges. The output 19.5 MHz pulses can be compressed from 11 ps to 542 fs and the maximum output power was 23.3 mW, corresponding to single pulse energy of 1.2 nJ. Numerical simulations approximately reflected the experimental results and revealed the mechanisms for self consistent intra-cavity pulse evolution.
2. Experiments and results
The experimental configuration of graphene oxide mode-locked dissipative soliton Er-doped fiber laser is shown in Fig. 1 . In the ring cavity, 3.5 m Er-doped fiber (EDF) with group velocity dispersion (GVD) of −13.2 ps/nm/km at 1530 nm and ~7 dB/m absorption was used as the gain medium, which pumped by a 974 nm diode laser. The cavity contained 1.5 m dispersion compensation fiber (DCF) with GVD of −130.8 ps/nm/km at 1530 nm, providing enough normal dispersion to balance the anomalous dispersion of single mode fiber (Corning SM28). The total cavity length was around 10.3 m and the net dispersion was about 0.19 ps2. An optical circulator was used to assures the unidirectional propagation of the laser and incorporate the graphene oxide saturable absorber mirror (GOSAM) into the cavity. The fiber was perpendicularly cleaved and butted to the GOSAM. The GOSAM used in the experiments was the same one used in our former work , which was made by depositing the graphene oxide hydrosol on a broadband reflective mirror. The Raman spectrum, saturable absorption curve and basic parameter (modulation depth of ~2.6%, non-saturable loss of ~30.5%, and saturation intensity of ~60 MW/cm2) were provided in . The polarization controller (PC) was used to adjust the polarization state. A 50% fiber coupler was used to output the signal. An optical spectrum analyzer (Yokogawa, AQ6370), an autocorrelator (Femtochrome, FR-103XL, resolution<5 fs), a 7.5 GHz radio-frequency analyzer (Agilent N900A-507), and a 25 GHz real-time oscilloscope (Agilent DSO-X92504A) with a 25 GHz photo-detector were employed to monitor the laser output simultaneously.
When the diode pump power increased to 139 mW, the self-starting stable pulses at a repetition rate of 19.5 MHz was achieved, and Fig. 2(a) shows the typical pulse train. The high threshold was caused by the large loss (~50%) between the fusion points of SMF and DCF. The black curve in Fig. 2(b) exhibits the optical spectrum with steep edges, which clearly indicates the mode-locked pulses in normal dispersion cavity have been shaped to dissipative soliton as a result of the mutual interaction among the laser gain and loss, cavity dispersion, fiber nonlinearity, and the birefringence filter effect induced by the polarization controller. The central wavelength was 1531 nm and spectral bandwidth was 6.5 nm. In this laser cavity with large splicing loss, the mode-locking operated in 1530 nm was easier to realize. The blue curve in Fig. 2(c) shows the measured autocorrelation trace obtained directly at the laser output port. The clean trace presents an autocorrelation width of 15.6 ps, which corresponding to pulse duration of 11 ps assuming a Gaussian pulse shape. The time-bandwidth product was calculated to be ~9.15, about 29 times more than the Fourier transform limit. According to the optical spectrum width, the Fourier transformed pulse width should be ~378 fs.
The pulse compression was realized by using single mode fiber (Corning SM28). The length of SMF was optimized under the direction of numerical stimulations, which are described in Section 3. The blue curve in Fig. 2(b) shows the optical spectrum of pulses propagated through 50 m SMF, where the pulse width was 3.3 ps. The red curve in Fig. 2(b) shows the spectrum through 75 m SMF, where the output pulses were externally compressed down to 542 fs (Autocorrelation width of 767 fs) and the measured autocorrelation trace was shown in Fig. 2(c) (red curve). According to the autocorrelation trace, the pedestal width was ~5 ps, so we estimated the energy in actual pulse was ~10% of the total. Large majority of energy still resided in wings, which can be improved by using a large-mode area fiber at extra-cavity to suppress the nonlinear pulse propagation . Because of the uncompensated third-order dispersion, the dechirped pulse width was larger than the Fourier transformed pulse width. Figure 2(d) shows the radio-frequency spectrum (after pulse compression) measured at a span of 200 kHz and a resolution bandwidth of 100 Hz. The fundamental peak located at the cavity repetition rate of 19.527 MHz has a signal-to-noise ratio of 65 dB.
The maximum output power (after pulse compression) was 23.3 mW at 500 mW pump power, corresponding to single pulse energy of 1.2 nJ. Neither pulse instability nor multiple pulse operation was observed. The maximum output power was limited by the obtainable pump power (500 mW). If the pump power could be increased, the maximum output power would be increased. Moreover, further investigation to improve the fusion efficiency will also enable the laser to deliver higher power pulses. The mode-locking operation was stable over several hours.
3. Numerical results
To study the intra-cavity pulse dynamics in this laser configuration and the extra-cavity pulse compression by single mode fiber, the numerical simulation based on extended Nonlinear Schrödinger equation (Eq. (1))  and split-step Fourier method  was investigated as reported in . The extended NLSE describes the temporal and longitudinal dependence of the slowly varying pulse envelope A (z, T) along each nonlinear dispersive element, where g is the distributed gain along the fiber, γ is the nonlinear parameter, β2 is the second-order dispersion and Ωg is the gain bandwidth. Higher-order nonlinear effects are neglected.
To be comparable with the experimental results, we used the following parameters for the current simulations. One cavity round trip included 3.5 m Er-doped fiber (β2 = 0.0168 ps2/m), 1.5 m DCF (β2 = 0.1668 ps2/m), and 5.3 m SMF (β2 = −0.0210 ps2/m). The gain saturates as Eq. (2), where g0 is 30 dB small-signal gain, E is the total energy of the pulse, and Esat is 10 nJ. The gain bandwidth was 20 nm with Gaussian profile. The saturable absorber is modeled by a reflectivity given in Eq. (3), where Runsat is the unsaturable reflectance, Rsat is the saturable reflectance, Psat is the saturable power and P is the instantaneous pulse peak power. The ratio of output coupler was 50%.
The evolution of pulse envelope and spectral distribution as a function of the propagation distance is shown in 2D-color maps (Fig. 3 ). The stable dissipative soliton can be obtained every time from quantum noise around 40 cavity round trips. The top picture in Fig. 4(a) shows the typical stimulated spectrum with steep edges and bandwidth of 6.1 nm. The simulated pulse duration (before compression) was 8.8 ps, shown in the top picture of Fig. 4(b). The numerical time-bandwidth product was 6.87. The tolerable deviations between the numerical and experimental results may be caused by the uncompensated high order dispersion and the polarization effects that have not been considered in the simulation.
The single mode fiber with anomalous dispersion was employed to remove the linear chirp of the dissipative soliton pulses. Figure 5 shows the relationship between the pulse width and the length of SMF. According to numerical simulations, the best compression effect was achieved at the length of 55 m, where the pulse width compressed from 8.8 ps to 386 fs. However, in our experiments we actually used 75 m SMF, because the experimental original pulse width was 11 ps, which was larger than the numerical pulse width of 8.8 ps. Figure 4(a) and Fig. 4 (b) respectively show the evolution of the output spectra and pulse profiles at different lengths of SMF (10, 30, 50, 55 and 65 m). The black curves in Fig. 4(a) were experimental spectra, which presented the similar shapes and variation tendency with the stimulation results.
We obtained stable 19.5 MHz pulse trains from a graphene oxide mode-locked Er-doped fiber laser that contained a segment of dispersion compensation fiber. The novel graphene oxide as a laser mode-locker has many merits, such as low cost, easy fabrication, ultrafast recovery time and hydrophilic property. The output pulses had pulse duration of 11 ps, compressible to 542 fs. The large positive chirp of the output pulses and the steep side edges of the pulse spectrum indicated dissipative soliton operation. The maximum output power was 23.3 mW, corresponding to single pulse energy of 1.2 nJ. The dynamics analysis by numerical stimulations provided theoretical explanation and support of these experimental results. Further investigation is currently under progress in order to explore energy scaling capabilities in this dissipative soliton laser.
The authors acknowledge the financial support from the National Nature Science Foundation of China (NSFC, Nos. 61177048), the Beijing Municipal Education Commission (No. KZ2011100050011) and Beijing University of Technology, China.
References and links
1. Q. L. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic layer graphene as saturable absorber for ultrafast pulsed laser,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
2. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, and K. P. Loh, “Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene,” Opt. Express 17(20), 17630–17635 (2009). [CrossRef] [PubMed]
3. H. Zhang, Q. Bao, D. Tang, L. Zhao, and K. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]
4. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]
5. D. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, and A. C. Ferrari, “Sub 200 fs pulse generation from a graphene mode-locked fiber laser,” Appl. Phys. Lett. 97(20), 203106 (2010). [CrossRef]
6. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). [CrossRef]
7. A. Martinez, K. Fuse, and S. Yamashita, “Mechanical exfoliation of graphene for the passive mode-locking of fiber lasers,” Appl. Phys. Lett. 99(12), 121107 (2011). [CrossRef]
8. P. L. Huang, S. C. Lin, C. Y. Yeh, H. H. Kuo, S. H. Huang, G. R. Lin, L. J. Li, C. Y. Su, and W. H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express 20(3), 2460–2465 (2012). [CrossRef] [PubMed]
9. B. V. Cunning, C. L. Brown, and D. Kielpinski, “Low-loss flake-graphene saturable absorber mirror for laser mode-locking at sub-200-fs pulse duration,” Appl. Phys. Lett. 99(26), 261109 (2011). [CrossRef]
10. L. Gui, W. Zhang, X. Li, X. Xiao, H. Zhu, K. Wang, D. Wu, and C. Yang, “Self-assembled graphene membrane as an ultrafast mode-locker in an erbium fiber laser,” IEEE Photon. Technol. Lett. 23(23), 1790–1792 (2011). [CrossRef]
11. Y.-W. Song, S.-Y. Jang, W.-S. Han, and M.-K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interaction,” Appl. Phys. Lett. 96(5), 051122 (2010). [CrossRef]
12. G. Sobon, J. Sotor, I. Pasternak, K. Grodecki, P. Paletko, W. Strupinski, Z. Jankiewicz, and K. M. Abramski, “Er-doped fiber laser mode-locked by CVD-graphene saturable absorber,” J. Lightwave Technol. 30(17), 2770–2775 (2012). [CrossRef]
13. H. Zhang, D. Tang, R. J. Knize, L. Zhao, Q. Bao, and K. P. Loh, “Graphene mode-locked, wavelength-tunable, dissipative soliton fiber laser,” Appl. Phys. Lett. 96(11), 111112 (2010). [CrossRef]
14. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh, B. Lin, and S. C. Tjin, “Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion,” Laser Phys. Lett. 7(8), 591–596 (2010). [CrossRef]
15. J. Xu, S. Wu, J. Liu, Q. Wang, Q. H. Yang, and P. Wang, “Nanosecond-pulsed erbium-doped fiber lasers with graphene saturable absorber,” Opt. Commun. 285(21–22), 326–329 (2012).
17. Z. B. Liu, X. Y. He, and D. N. Wang, “Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution,” Opt. Lett. 36(16), 3024–3026 (2011). [CrossRef] [PubMed]
18. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, and K. M. Abramski, “Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser,” Opt. Express 20(17), 19463–19473 (2012). [CrossRef]
19. J. Liu, Y. G. Wang, Z. S. Qu, L. H. Zheng, L. B. Su, and J. Xu, “Graphene oxide absorber for 2 µm passive mode-locking Tm: YAlO3 laser,” Laser Phys. Lett. 9(1), 15–19 (2012). [CrossRef]
20. L. Zhang, Y. G. Wang, H. J. Yu, S. B. Zhang, W. Hou, X. C. Lin, and J. M. Li, “High power passively mode-locked Nd:YVO4 laser using graphene oxide as a saturable absorber,” Laser Phys. 21(12), 2072–2075 (2011). [CrossRef]
21. W. S. Hummers and R. E. Offeman, “Preparation of graphite oxide,” J. Am. Chem. Soc. 80(6), 1339 (1958). [CrossRef]
22. S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. Nguyen, and R. Ruoff, “Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide,” Carbon 45(7), 1558–1565 (2007). [CrossRef]
23. D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, and R. S. Ruoff, “Preparation and characterization of graphene oxide paper,” Nature 448(7152), 457–460 (2007). [CrossRef] [PubMed]
24. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]
25. X. Zhao, Z. B. Liu, W. B. Yan, Y. Wu, X. L. Zhang, Y. Chen, and J. G. Tian, “Ultrafast carrier dynamics and saturable absorption of solution-processable few-layered graphene oxide,” Appl. Phys. Lett. 98(12), 121905 (2011). [CrossRef]
27. P. Grelu and N. Akhmediev, “Dissipative solitons for mode-locked lasers,” Nat. Photonics 6(2), 84–92 (2012). [CrossRef]
28. J. H. Im, S. Y. Choi, F. Rotermund, and D. I. Yeom, “All-fiber Er-doped dissipative soliton laser based on evanescent field interaction with carbon nanotube saturable absorber,” Opt. Express 18(21), 22141–22146 (2010). [CrossRef] [PubMed]
29. V. I. Kruglov, C. Aguergaray, and J. D. Harvey, “Parabolic and hyper-Gaussian similaritons in fiber amplifiers and lasers with gain saturation,” Opt. Express 20(8), 8741–8754 (2012). [CrossRef] [PubMed]
30. E. Ding and J. N. Kutz, “Operating regimes, split-step modeling, and the Haus master mode-locking model,” J. Opt. Soc. Am. B 26(12), 2290–2300 (2009). [CrossRef]
31. T. Schreiber, B. Ortaç, J. Limpert, and A. Tünnermann, “On the study of pulse evolution in ultra-short pulse mode-locked fiber lasers by numerical simulations,” Opt. Express 15(13), 8252–8262 (2007). [CrossRef] [PubMed]