Graphene oxide mode-locked femtosecond erbium-doped fiber lasers

Graphene oxide mode-locked femtosecond erbium-doped fiber lasers Jia Xu, 1 Jiang Liu, 1 Sida Wu, 2 Quan-Hong Yang, 2 and Pu Wang 1,* 1 Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China 2 School of Chemical Engineering and Technology, Tianjin University, qhyangcn@tju.edu.cn, Tianjin 300072, China * wangpuemail@bjut.edu.cn Abstract: We demonstrated the femtosecond erbium-doped all-fiber lasers mode-locked with graphene oxide, which can be conveniently obtained from natural graphite by simple oxidation and ultra-sonication process. With proper dispersion management in an all-fiber ring cavity, the laser directly generated 200 fs pulses at a repetition rate of 22.9 MHz and the average output power was 5.8 mw. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. These results are comparable with those of graphene saturable absorbers and the superiority of easy fabrication and hydrophilic property of graphene oxide will facilitate its potential applications for ultrafast photonics. 2012 Optical Society of America OCIS codes: (140.3500) Lasers, erbium; (140.4050) Mode-locked lasers; (140.7090) Ultrafast lasers; (160.4330) Nonlinear optical materials. References and links 1. H. P. Sardesai, C. C. Chang, and A. M. Weiner, A femtosecond code-division multiple-access communication system test bed, J. Lightwave Technol. 16(11), 1953 1964 (1998). 2. N. Nishizawa, Y. Chen, P. Hsiung, E. P. Ippen, and J. G. Fujimoto, Real-time, ultrahigh-resolution, optical coherence tomography with an all-fiber, femtosecond fiber laser continuum at 1.5 microm, Opt. Lett. 29(24), 2846 2848 (2004). 3. T. Udem, R. Holzwarth, and T. W. Hänsch, Optical frequency metrology, Nature 416(6877), 233 237 (2002). 4. J. W. Nicholson, A. D. Yablon, P. S. Westbrook, K. S. Feder, and M. F. Yan, High power, single mode, allfiber source of femtosecond pulses at 1550 nm and its use in supercontinuum generation, Opt. Express 12(13), 3025 3034 (2004). 5. T. Hasan, Z. P. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, Nanotube-polymer composites for ultrafast photonics, Adv. Mater. (Deerfield Beach Fla.) 21(38 39), 3874 3899 (2009). 6. 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). 7. Y. Song, S. Jang, W. Han, and M. Bae, Graphene mode-lockers for fiber lasers functioned with evanescent field interaction, Appl. Phys. Lett. 96(5), 051122 (2010). 8. A. Martinez, K. Fuse, and S. Yamashita, Mechinecal exfoliation of graphene for the passive mode-locking of fiber lasers, Appl. Phys. Lett. 99(12), 121107 (2011). 9. Q. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Tang, and K. P. Loh, Monolayer graphene as a saturable absorber in a mode-locked laser, Nano Res. 4(3), 297 307 (2011). 10. 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). 11. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh, B. Lin, and S. C. Tjin, Compact grapheme modelocked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion, Laser Phys. Lett. 7(8), 591 596 (2010). 12. 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). 13. Q. L. Bao, H. Zhang, J. Yang, S. Wang, D. Y. Tang, R. Jose, S. Ramakrishna, C. T. Lim, and K. P. Loh, Graphene-polymer nanofiber membrane for ultrafast photonics, Adv. Funct. Mater. 20, 1 10 (2010). 14. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4(9), 611 622 (2010). 15. A. Martinez, K. Fuse, B. Xu, and S. Yamashita, Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing, Opt. Express 18(22), 23054 23061 (2010). 16. Y. M. Chang, H. Kim, J. H. Lee, and Y. W. Song, Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers, Appl. Phys. Lett. 97(21), 211102 (2010). (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15474

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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). 26. 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). 27. K. P. Loh, Q. L. Bao, G. Eda, and M. Chhowalla, Graphene oxide as a chemically tunable platform for optical applications, Nat. Chem. 2(12), 1015 1024 (2010). 28. W. S. Hummers and R. E. Offeman, Preparation of graphite oxide, J. Am. Chem. Soc. 80(6), 1339 1339 (1958). 29. 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: YAlO 3 laser, Laser Phys. Lett. 9(1), 15 19 (2012). 30. M. L. Dennis and I. N. Duling, Experimental study of sideband generation in femtosecond fiber lasers, Quantum Electron. 30(6), 1469 1477 (1994). 31. L. Gui, X. Yang, G. Zhao, X. Yang, X. Xiao, J. Zhu, and C. Yang, Suppression of continuous lasing in a carbon nanotube polyimide film mode-locked erbium-doped fiber laser, Appl. Opt. 50(1), 110 115 (2011). 1. Introduction Femtosecond erbium-doped fiber lasers have many applications in various industrial and scientific research areas, such as optical communications [1], optical coherent tomography [2], optical atomic clock [3] and supercontinuum generation [4]. Passive mode-locking is a practical technique to generate ultrafast femtosecond pulses in highly compact fiber lasers. Recently, nano-material graphene-based saturable absorber has attracted considerable interest as an excellent wideband mode-locker due to its unique linear and nonlinear optical properties, such as saturable absorption characteristics for a broad wavelength range, ultrafast recover time, low saturable intensity, and pulses from sub 200 fs to a few picoseconds in the graphene mode-locked erbium-doped fiber lasers have been reported [5 20]. In 2004, Novoselov et al. first produced graphene by mechanical exfoliation [21], although this method suffers from the ultra-low success ratio. And then various methods for high quality, large scale fabrication of graphene are actively explored, such as chemical vapor deposition (CVD) [22], thermal decomposition from SiC [23], and chemical reduction method [24]. The chemical reduction method involves complex chemical processes and generates graphene with heavily functionalized organic groups. The first step of chemical reduction method is to synthesize graphene oxide from natural graphite powder. Then graphene-based nanosheet can be obtained from graphene oxide by chemical methods using reductants such as hydrazine, dimethylhydrazine. Graphene oxide, served as the precursor for graphene, has also been widely investigated for its own physical and chemical characteristics. On one hand, the presence of oxygen-containing functional groups makes graphene oxide strongly hydrophilic and water soluble, which is different with graphene. The solubility offers superior flexibility and processibility for large-scale production of graphene oxide based optoelectronics. For example, we can fabricate grapheneoxide membrane on different kinds of substrates by a spin-coater, or inject graphene oxide solution into a hollow-core photonic crystal fiber [25]. On the other hand, although the (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15475

oxygen functional groups destroy the gapless linear dispersion of Dirac electrons in graphene and make graphene oxide insulating, it has been demonstrated that graphene oxide also has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of graphene [26,27]. These properties make graphene oxide as potential saturable absorber material in pulsed fiber lasers. Up to now, there are two reports of graphene oxide mode-locked fiber lasers. In 2010, F. Bonaccorso et al. reported the graphene oxide modelocked fiber laser for the first time [14]. However, in this paper they only provided the autocorrelation trace and optical spectrum of ~743 fs pulses, and other important information were not given, such as the cavity configuration, pulse trains and nonlinear optical parameter of graphene oxide. In 2011, Liu et al. generated pulsed erbium-doped fiber laser based on a hollow-core photonic crystal fiber filled with graphene oxide solution [25], but the pulse width was 4.85 ns. Here, we report femtosecond mode-locked erbium-doped fiber lasers, which adopted ring cavity configuration and self-assembled graphene oxide saturable absorber mirror. With dispersion management, the laser directly generated 200 fs pulses at a repetition rate of 22.9 MHz and the average output power was 5.8 mw. To the best of our knowledge, 200 fs is the shortest pulses obtained from graphene-oxide-based fiber laser. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. Considering the outstanding advantages of low price, easy fabrication and amphipathic properties, the graphene oxide is promising candidate as saturable absorber and can be used as practical and efficient photonic material for generation of ultrafast fiber lasers. 2. Preparation of the graphene oxide saturable absorber mirror (GOSAM) The graphite oxide was synthesized from natural graphite powder by a modified Hummers method [28]. The graphene oxide hydrosol with concentration of 2 mg/ml was prepared by ultrasonic peeling of graphite oxide in aqueous suspension. Then a broadband reflective mirror was immersed into the graphene oxide hydrosol for 48 hr. Finally, a thin graphene oxide membrane was formed on the broadband reflective mirror. Figure 1(a) shows the Raman spectrum of the graphene oxide membrane, which was excited by a 514 nm Ar ion laser. The Raman spectrum reveals two prominent features of graphene oxide at 1347 cm 1 and 1593 cm 1, which are assigned to D and G bands, respectively. The D band is from the structural imperfections created by the attachment of hydroxyl and epoxide groups on the carbon basal plane. The G band corresponds to ordered sp 2 bonded carbon [29]. According to the reports in [6] and [10], there was an obvious band around 2700 cm 1 named 2D band in the Raman spectrum of graphene, which is considered as an evident feature of graphene material. In Fig. 1(a), the 2D band was hardly observed, which indicates there was no graphene on GOSAM. Figure 1(b) shows the measured reflection of GOSAM at different incident power using a probe laser with ~600 fs pulse width at 38 MHz repetition rate. This source was achieved by a SESAM mode-locked erbium-doped fiber laser and 10% of the output beam was used to monitor the input power, while the 90% was used to pump the GOSAM. The modulation depth of GOSAM was ~2.6% at 1558 nm. The insert loss of GOSAM and three-port circulator was ~60.5% in total, shown in Fig. 1(b). The loss of the three-port circulator measured to be ~30%, so the non-saturable loss of the GOSAM was ~30.5%. The saturable incident power was ~0.7 mw, corresponding to saturation intensity of ~60 MW/cm 2. In [10], Sun et al. inserted graphene membrane between two FC/APC fiber connectors to generate mode-locked pulses. In this way, the modulation depth was measured to be 1.3%, the nonsaturable loss was 34.3% and the saturation intensity was 266 MW/cm 2. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15476

Fig. 1. (a) Raman spectrum of graphene oxide. (b) Saturable absorption of graphene oxide. 3. Experimental results and discussions 3.1 Graphene oxide mode-locked fiber laser in anomalous dispersion cavity The experimental configuration of femtosecond graphene oxide mode-locked erbium-doped fiber laser is shown schematically in Fig. 2. The ring cavity included a piece of 1 m erbiumdoped fiber and ~6.8 m single mode fiber. The cavity length was around 7.8 m, and the net dispersion was estimated to be 0.14 ps 2. The erbium-doped fiber with ~7 db/m absorption was core pumped by a diode laser with a center wavelength of 974 nm and a maximum output power of 600 mw. An optical circulator was used to incorporate the graphene oxide saturable absorber mirror (GOSAM) into the cavity. The fiber of circulator 2nd-port was perpendicularly cleaved and butted to the GOSAM. A 30% fiber coupler was used to output the signal. An optical spectrum analyzer (Yokogawa, AQ6370), a 7.5GHz 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. Fig. 2. Schematic setup of the graphene oxide mode-locked erbium-doped fiber laser. WDM: wavelength division multiplexer; SMF: single mode fiber; GOSAM: graphene oxide saturable absorber mirror. When the diode pump power increased to 33 mw, the self-started mode-locking occurred. Figure 3(a) shows a typical pulse train at repetition rate of 25.6 MHz, which corresponds to the total cavity length of ~7.8 m. The spectral FWHM of 5.4 nm was centered at 1556.9 nm, measured by an optical spectral analyzer with resolution of 0.02 nm (Fig. 3(b)). Figure 3(c) shows a typical autocorrelation trace, which is well fitted by a sech 2 temporal profile, resulting in pulse duration of ~600 fs. The time-bandwidth product (TBP) was 0.405 at fundamental soliton-like operation, conformed by the clearly visible Kelly sidebands of optical spectrum [30]. The maximum output power was 3.3 mw at 98 mw pump power, corresponding to single pulse energy of 0.13 nj and peak power of 220 W. Further increase of the pump power, the wave breaking occurred. Eventually, harmonic mode-locking with two times of the fundamental frequency was also observed. The radio-frequency spectrum (Fig. 3(d)) shows its fundamental peak located at the cavity repetition rate of 25.6 MHz, with a signal-to-noise ratio of 50 db, indicating good mode-locking stability. To verify that the mode locking resulted from the graphene oxide, we purposely used a broadband mirror to replace the GOSAM from the cavity and then no mode locking was observed. In this work, the stability performance of the fiber laser was monitored for 8 hours. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15477

Fig. 3. Characterization of graphene oxide mode-locked fiber laser in anomalous dispersion cavity: (a) Stable pulse train at 25.6 MHz repetition rate. (b) Optical spectrum. (c). 600 fs pulse width. (d) Frequency spectrum. 3.2 Graphene oxide mode-locked fiber laser in near zero dispersion cavity According to the dispersion management theory, shorter pulses can be achieved by adjusting the lengths of erbium-doped fiber and single mode fiber to give near zero round trip group velocity dispersion. In this experiment, we increased the length of erbium-doped fiber with GVD parameter of 11.7 (ps/nm/km) from 1 m to 3 m, in order to compensate the negative dispersion of single mode fiber. The single mode fiber length was optimized to be 5.7 m to get stable pulse trains, and the total dispersion was calculated to be 0.088 ps 2, which was closer to zero than 0.14 ps 2 in last work. Meanwhile, a 70% fiber coupler was used to increase the output power. The stable mode-locking occurred at 27 mw pump power. Figure 4(a) shows a typical pulse train at repetition rate of 22.9 MHz, Fig. 4(c) shows the 200 fs pulse width under sech 2 assumption, and Fig. 4(b) shows the optical spectrum centered at 1560 nm. The central wavelength had a slightly red shift, which means the gain of the whole cavity was increased. The gain caused by longer erbium-doped fiber overweighed the loss of a coupler with higher output radio. There was an obvious continuous wave component (the narrow peak at the center of the spectrum) existing, which was caused by an insufficient modulation depth of GOSAM [31]. In [31], Gui et al. demonstrated the saturable absorber with larger modulation depth can suppress the continuous wave of mode-locked pulses. So we used a SESAM with 34% modulation depth in this cavity configuration and obtained ~200 fs pulse train without continuous wave component. That means if we can fabricate a larger modulation depth GOSAM, we can suppress the continuous wave component and the mode-locking stability will also greatly increased. Figure 4(d) shows the radio-frequency spectrum measured at a span of 4 khz and a resolution bandwidth of 10 Hz. The fundamental peak located at the cavity repetition rate of 22.9 MHz has a signal-to-noise ratio of 60 db. The maximum output power was 5.8 mw at 69 mw pump power. Further increase of the pump power, the pulse breaking occurred. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15478

Fig. 4. Characterization of graphene oxide mode-locked fiber laser in near zero dispersion cavity: (a) Stable pulse train at 22.9 MHz repetition rate. (b) Optical spectrum. (c) 200 fs pulse width. (d) Frequency spectrum. In order to investigate the dependence of pulse width and net cavity dispersion, the length of single mode fiber was varied while the 3 m erbium-doped fiber maintained. Table 1 included six groups of data shows the relationship between cavity design and laser performance. When the net dispersion varied from 0.088 ps 2 to 4.9 ps 2, we generated pulses with pulse width of 200 fs ~3 ps at repetition rate of 23 MHz ~930 khz. Table 1. Optical Parameter of Graphene Oxide Mode-locked Fiber Laser Cavity length (m) Total dispersion (ps 2 ) Frequency (MHz) Pulse width (ps) 8.7 0.088 23 0.2 11.8 0.16 17 0.6 27 0.52 7.4 1.2 69 1.5 2.9 1.8 160 3.6 1.25 2.6 215 4.9 0.93 3 Compared with the reports [6,9], the mode-locking performance (such as stability, pulse width, spectrum shape) of the fiber lasers based on graphene oxide was as good as that of atomic-layer graphene. As we known, atomic-layer graphene has much better performance in mode-locking than multi-layer graphene because of larger modulation depth, but the current approaches cannot satisfy the large yields, layer controlled production of graphene. Monolayer graphene oxide can be easily peeled from graphite oxide by a simple ultrasonication process, and the oxygen groups attached on the graphene oxide nanosheet provide good hydrophilic properties, making it easy for large-scale production. By further optimization of the cavity design and improvement on the GOSAM, we could generate modelocked pulses with narrower pulse width and larger pulse energy. 4. Conclusion In summary, we have demonstrated femtosecond graphene oxide mode-locked erbium-doped fiber laser. With dispersion management, the total dispersion can be decreased to 0.088 ps 2, where the pulse width was 200 fs at 22.9 MHz repetition rate and the average output power was 5.8 mw. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. The superiority of easy fabrication and strong solubility will facilitate potential applications of graphene oxide for ultrafast photonics. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15479

Acknowledgment 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. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15480