Open Access
Add to BibSonomyAbstract
Using a two-layer structure consisting of polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) to support graphene grown by chemical vapor deposition (CVD), we demonstrate a flexible integrated graphene saturable absorber (SA) on microfiber for passive mode-locked soliton fiber laser. This method can optimize the light-graphene interaction by using evanescent field in the integration structure. Moreover, the fiber laser with the in-line microfiber-to-graphene SA can realize the tunabilities of both the 3dB bandwidth of output optical spectrum and the pulse width of soliton. This tunable mode-locked soliton laser has potential applications in optical communication, optical microscopy, and so on.
© 2014 Optical Society of America
1. Introduction
Owing to the great potential applications in spectroscopy, microscopy, biomedical research, optical communication and so forth, an increasing number of attentions have been paid to the passive mode-locked fiber laser based on saturable absorber (SA) [1–4]. Conventionally, semiconductor saturable absorber mirrors (SESAMs) and single-walled carbon nanotubes (SWNTs) are applied as SA. However, SESAMs are usually considered as expensive and complicated-fabrication devices [5, 6]. Uncontrollable diameter and chirality of SWNTs makes it difficult to obtain a desirable energy bandgap [7–9]. Compared with them, graphene is a better substitute of SA because of its nature of linear dispersion of massless Dirac like particles [10–12]. The graphene SA is capable of providing faster recovery time, larger modulation depth and broader band operation [4, 13]. At present, ultrafast mode-locked fiber laser based on graphene and its hybrid materials have been investigated widely [4, 14–21]. The graphene SA is usually transferred to fiber adaptors to let powerful incident light directly penetrate into graphene layers. In addition, graphene combined with side-polished fiber, tapered fiber, and hollow photonic crystal fiber has also been used as a SA in mode-locked fiber laser [22–24]. However, it is still difficult to control the uniformity of graphene and optimize the light-graphene interaction in order to obtain better mode-locking performance.
In this paper, an optimized SA consisting of single-layered CVD growth graphene, supported by a flexible film of PET/PDMS, and microfiber is presented. The results show obvious polarization dependent saturable absorption in this graphene SA. Since the performances of mode-locked fiber laser is closely related with the saturable absorption of SA, the polarization dependent saturable absorption implies a tunable passive mode-locked fiber laser can be realized by adjusting the polarization state of light in fiber. Here, we obtained a tunable femtosecond pulse laser with the changes of pulse width (~160 fs) and optical spectrum (~7.6 nm) using the microfiber based graphene SA. The good characteristics of tunability, stability, compactness and all-fiber configuration and the flexible graphene integrated method will promote the application of graphene mode-locked fiber lasers.
2. Fabrication, characteristic and polarization measurement of microfiber based SSA
The preparation process of this graphene-based SA is shown in Fig. 1(a). Firstly, we pour the 1:1 mixture of paraxylene and PDMS onto a PET substrate and spin it. The film is then cured for 7 hours in vacuum to obtain a ~20 µm thick PDMS. Paraxylene was used to dilute the PDMS to make the obtained film thinner and more uniform. Additionally, in order to reduce contamination, we apply a dry transfer method to transfer a single-layer CVD graphene to the PET/PDMS film [25]. Figures 2(a)-2(c) present the photograph of the sample and the scanning electron microscope (SEM) image, Raman spectrum of graphene. The Raman spectrum is measured at 514 nm. The G band at 1583 cm−1 and 2D band at 2683 cm−1, the second order of the D band at 1346 cm−1, can be found easily. They relate to the phonon scattering at the center of Brillouin zone and phonon activation respectively. The higher 2D/G band intensity ratio and a weaker D band both suggest low density of defects of the CVD graphene which is prepared taking advantage of a cleaner method [26]. We then use two PET/PDMS/graphene films to sandwich a microfiber with a waist diameter of ~7.5 µm, a stretching length of 20 mm, and an insertion loss of 0.1 dBm. Finally, the sandwich-type graphene SA (SSA) can be made, as shown in Figs. 1(b)-1(c).
Fig. 1 Preparation of an optimized SSA based on microfiber. (a) Schematic diagram of the experimental procedure for manufacturing the microfiber based SSA. (b) and (c) Pictures of the flexible PDMS/PET film and the SSA.
Fig. 2 (a) Photograph of PET/PDMS/graphene structure. (b) and (c) SEM image and Raman spectrum of graphene sample.
We fabricated three different samples (Sample A, B and C) by using the above preparation process. Sample A consists of a microfiber with a waist diameter of 7.28 µm sandwiched between two 4 × 6 mm2 graphene supported by PET/PDMS films. Its insertion loss measured by unpolarized light is 11.3 dB. For comparison, one PET/PDMS film without graphene and one PET/PDMS/graphene film were used to sandwich the microfiber for sample B and sample C. The waist diameter of microfiber in sample B and C are 7.60 and 7.51 µm. Meanwhile, the sizes of graphene are 4 × 6 and 4 × 10 mm2, respectively. As a result, their insertion losses are 5.5and 10.8 dB.
The polarization-dependent saturable absorption as shown in Fig. 3 is measured by a home-made mode-locked laser with 1.5 ps pulse at 1.55 µm [27]. We found that the transmission (TTE) of TE mode of sample A is almost 20.7 times higher than that (TTM) of the TM mode in the nonsaturable range, which is much larger than sample B (TTE/TTM = 1.9) and C (TTE/TTM = 4.4). The linear absorptions of TE and TM modes are 10.32 and 23.41 dB for sample A, 4.14 and 6.90 dB for sample B, and 9.34 and 15.69 dB for sample C, respectively. The polarization dependence of this SSA is so sensitive that we can regulate the mode-locked pulse expediently. While, it is notable that the linear absorption is correlated with the size of graphene for sample B and C. Almost the same ratio of linear absorption (9.34/4.14 = 2.256, 15.69/6.90 = 2.274) indicate that we can easily control the intensity of optical absorption of graphene SA by changing the size of graphene.
Fig. 3 The transmittance of TE and TM modes for three samples as a function of incident peak power density.
In this SSA, the evanescent field of the guided mode propagating along the microðber can effectively interact with graphene. As the incident light propagates along the microfiber, it can be found that the loss of TM mode is greater than that of TE mode. That means the absorption in parallel and perpendicular polarization is different (dichroism). Due to the carriers in graphene is closer to massless Dirac fermions, thus the dynamic conductivity of graphene (intraband and interband conductivity) defined by Kubo formalism must be modified. According to previous reports, both the intraband and interband conductivity are plural forms for TE and TM modes in graphene, and the imaginary portion of which will directly affect the attenuation of two polarization modes [28]. When the chemical potential of graphene is lower, the negative imaginary part of interband conductivity plays a leading role, which results in the improvement of the transmission of TE mode [27–30]. Since the CVD graphene sandwich the microfiber close in this SSA, sensitive polarization dependence of saturable absorption can be got. Thus, the control of light polarization states provides an efficient way of tuning saturable absorption or modulation depth of the graphene-based SA.
3. Experimental setup and results discussion
Figure 4 presents the experimental setup of the SSA based mode-locked fiber laser. The ringed resonator is composed of a 980/1550 nm single-mode Wavelength Division Multiplex (WDM) coupler, a 1-m high-doped erbium doped fiber (Thorlabs EDF-80), a fiber isolator (ISO), a polarization controller (PC), a 10% output coupler and the SSA which is pumped by a 980nm laser diode (LD). The whole cavity length is 6.8 m with −0.094 ps2 anomalous dispersion. It is beneficial to produce conventional soliton due to the influence of negative group velocity dispersion (GVD), self-phase modulation (SPM), nonlinear optical Kerr effect (NKE) and other nonlinear effects in fiber [31]. Certainly, the dissipative solitons (DS) with non-sideband can also be achieved in net normal dispersion systems [32, 33]. Adjusting the orientation of PC, we can tune the quality of the pure anomalous dispersion soliton including the pulse intensity and width.
The output laser performance is measured by a 200 MHz bandwidth photodetector (KG-DR), a 1GHz digital oscilloscope (Yokogawa DL9140), an optical spectrum analyzer (Yokogawa AQ6370) and an interferometric autocorrelator (FR-103XL). When the pump power rises up to 100 mW, stable mode-lock soliton states can be got. By this time, changing the direction of PC slightly, under the premise of nondestructive soliton, the shape of soliton can be modified. The strong Kelly sidebands of spectrum imply the presence of soliton [34]. During the modulating process, with the 3dB spectral bandwidth narrowing, the peak wavelength has a slight change and the full width at half maximum (FWHM) of soliton becomes wider at the same time, which is consistent with previous work [32]. This is because the different interaction intensity of TE and TM mode with graphene and self-steeping effect (SSE), Raman-induced frequency shift (RIFS) of soliton when it propagates in fiber [28, 35–37].
For sample B, the 3dB spectral bandwidth is variable from ~4.5 to~7.4 nm and the relevant FWHM of soliton changes from ~522 to ~459 fs. The center of the spectral only changes from 1564 to 1566 nm and the intensity of soliton decreases ~1.15 times (from 1.8 to 1.56 as shown in Figs. 7(a) and 7(b)). Since the polarization-dependent absorption is stronger in sample C than sample B, a lager change of the soliton pulse can be obtained for sample C. When the 3dB spectral bandwidth can be tuned from ~2.0 to ~8.1 nm, the FWHM of soliton changes from ~550 to ~450 fs. The center of the spectral is from 1552 to 1558 nm and the intensity of soliton decreases mightily (from 1.68 to 0.09 as shown in Figs. 6(a)-6(b)). Compared with sample B and sample C, the regulation range of sample A is the most obvious on account of the particular sandwich structure. Figures 5(a)-(b) displays the output characteristics of the mode-locked fiber laser using sample A. By adjusting PC, 3dB spectral bandwidth changes in a range of ~2.4 - ~10.0 nm and the FWHM changes in a range of ~550 - ~390 fs. At the meantime, the power of soliton drops rapidly (from 1.56 to 0.102) and the center of the spectrum changes from ~1563 to ~1557 nm. The small changes of peak wavelength for three samples indicate the output soliton has good stability of spectrum. For a polarization-independent SA (e.g. carbon nanotube SA), however, a remarkable tuning range of output spectrum can be achieved by using the filter components (band-pass filter or chirped fiber Bragg grating) in laser cavity [19, 38] (See Fig. 6 and Fig. 7).
Fig. 5 Mode-locked soliton pulses output performance of sample C. (a) and (b) Tunable 3 dB spectral bandwidth from 2.04 to 8.13 nm and FWHM of autocorrelation trace from 556 to 450 fs.
Fig. 6 Mode-locked soliton pulses output performance of sample A. (a) and (b) Tunable 3 dB spectral bandwidth from 2.44 to 10.02 nm and FWHM of autocorrelation trace from 555 to 390 fs.
Fig. 7 Mode-locked soliton pulses output performance of sample B. (a) and (b) Tunable 3 dB spectral bandwidth from 4.49 to 7.43 nm and FWHM of autocorrelation trace from 522 to 459 fs.
In addition, owing to the repetition frequency (f) of soliton pulse is determined by the cavity length (L) and the refractive index of fiber (n), the pulse train is constant with a period of 29.5 ns even though we change the sample and laser modes (Figs. 8(a)-8(c)). It is visible that the calculated repetition frequency, where c is the speed of light in vacuum, is accordant with the whole cavity length well.
4. Conclusions
In summary, we have reported a tunable passive mode-locked soliton fiber laser based on microfiber and CVD graphene which is supported by a flexible two-layer film composed of PET and PDMS. By making use of the strong polarization dependence of the saturable absorption, we can tune the 3dB spectral bandwidth from ~2.4 to ~10 nm, and FWHM of soliton pulse width changes with a range of 160 fs. Meanwhile, the peak wavelength of soliton only has a small change. Because of these good performances, it is promising that this tunable laser based on the optimized SSA can be considered for applications in many fields.
Acknowledgments
The authors thank The Chinese National Key Basic Research Special Fund (Grant 2011CB922003), the Natural Science Foundation of China (Grant 11174159, 11374164), and the Natural Science Foundation of Tianjin (Grant 13JCYBJC16300).
References and links
1. C. Xu and F. W. Wise, “Recent advances in fibre lasers for nonlinear microscopy,” Nat. Photonics 7(11), 875–882 (2013). [CrossRef] [PubMed]
2. A. Martinez and Z. Sun, “Nanotube and graphene saturable absorbers for fibre lasers,” Nat. Photonics 7(11), 842–845 (2013). [CrossRef]
3. M. E. Fermann and I. Hartl, “Ultrafast fibre lasers,” Nat. Photonics 7(11), 868–874 (2013). [CrossRef]
4. 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 a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
5. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]
6. O. Okhotnikov, A. Grudinin, and M. Pessa, “Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications,” New J. Phys. 6(1), 177 (2004). [CrossRef]
7. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–polymer composites for ultrafast photonics,” Adv. Mater. 21(38–39), 3874–3899 (2009). [CrossRef]
8. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, “Wideband-tuneable, nanotube mode-locked, fibre laser,” Nat. Nanotechnol. 3(12), 738–742 (2008). [CrossRef] [PubMed]
9. Y. Senoo, N. Nishizawa, Y. Sakakibara, K. Sumimura, E. Itoga, H. Kataura, and K. Itoh, “Polarization-maintaining, high-energy, wavelength-tunable, Er-doped ultrashort pulse fiber laser using carbon-nanotube polyimide film,” Opt. Express 17(22), 20233–20241 (2009). [CrossRef] [PubMed]
10. A. K. Geim, “Graphene: status and prospects,” Science 324(5934), 1530–1534 (2009). [CrossRef] [PubMed]
11. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, and R. S. Ruoff, “Graphene and graphene oxide: synthesis, properties, and applications,” Adv. Mater. 22(35), 3906–3924 (2010). [CrossRef] [PubMed]
12. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef] [PubMed]
13. G. Xing, H. Guo, X. Zhang, T. C. Sum, and C. H. A. Huan, “The Physics of ultrafast saturable absorption in graphene,” Opt. Express 18(5), 4564–4573 (2010). [CrossRef] [PubMed]
14. 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]
15. 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). [CrossRef]
16. 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] [PubMed]
17. 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]
18. A. Martinez and S. Yamashita, “10 GHz fundamental mode fiber laser using a graphene saturable absorber,” Appl. Phys. Lett. 101(4), 041118 (2012). [CrossRef]
19. Z. Sun, D. Popa, T. Hasan, F. Torrisi, F. Wang, E. J. Kelleher, J. C. Travers, V. Nicolosi, and A. C. Ferrari, “A stable, wideband tunable, near transform-limited, graphene-mode-locked, ultrafast laser,” Nano Res. 3(9), 653–660 (2010). [CrossRef]
20. Q. Wang, T. Chen, B. Zhang, M. Li, Y. Lu, and K. P. Chen, “All-fiber passively mode-locked thulium-doped fiber ring laser using optically deposited graphene saturable absorbers,” Appl. Phys. Lett. 102(13), 131117 (2013). [CrossRef]
21. X. Li, Y. Wang, Y. Wang, Y. Zhang, K. Wu, P. Shum, X. Yu, Y. Zhang, and Q. Wang, “All-normal-dispersion passively mode-locked Yb-doped fiber ring laser based on a graphene oxide saturable absorber,” Laser Phys. Lett. 10(7), 075108 (2013). [CrossRef]
22. 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]
23. Z. B. Liu, X. 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]
24. J. Wang, Z. Luo, M. Zhou, C. Ye, H. Fu, Z. Cai, H. Cheng, H. Xu, and W. Qi, “Evanescent-light deposition of graphene onto tapered fibers for passive Q-switch and mode-locker,” IEEE Photonics J. 4(5), 1295–1305 (2012). [CrossRef]
25. X. D. Chen, Z. B. Liu, C. Y. Zheng, F. Xing, X. Q. Yan, Y. S. Chen, and J. G. Tian, “High-quality and efficient transfer of large-area graphene films onto different substrates,” Carbon 56, 271–278 (2013). [CrossRef]
26. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene layers,” Phys. Rev. Lett. 97(18), 187401 (2006). [CrossRef] [PubMed]
27. Q. W. Sheng, M. Feng, W. Xin, T. Y. Han, Y. G. Liu, Z. B. Liu, and J. G. Tian, “Actively manipulation of operation states in passively pulsed fiber lasers by using graphene saturable absorber on microfiber,” Opt. Express 21(12), 14859–14866 (2013). [CrossRef] [PubMed]
28. Q. L. Bao, H. Zhang, B. Wang, Z. H. Ni, C. Haley, Y. Wang, D. Y. Tang, and K. P. Loh, “Broadband graphene polarizer,” Nat. Photonics 5(7), 411–415 (2011). [CrossRef]
29. X. He, Z. B. Liu, D. Wang, M. Yang, T. Y. Hu, and J. G. Tian, “Saturable absorber based on graphene-covered-microfiber,” IEEE Photon. Technol. Lett. 25(14), 1392–1394 (2013). [CrossRef]
30. J. T. Kim and C. G. Choi, “Graphene-based polymer waveguide polarizer,” Opt. Express 20(4), 3556–3562 (2012). [CrossRef] [PubMed]
31. S. L. McCall and E. L. Hahn, “Self-induced transparency by pulsed coherent light,” Phys. Rev. Lett. 18(21), 908–911 (1967). [CrossRef]
32. 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]
33. X. M. Liu, “Hysteresis phenomena and multipulse formation of a dissipative system in a passively mode-locked fiber laser,” Phys. Rev. A 81(2), 023811 (2010). [CrossRef]
34. S. Kelly, “Characteristic sideband instability of periodically amplified average soliton,” Electron. Lett. 28(8), 806–807 (1992). [CrossRef]
35. N. Tzoar and M. Jain, “Self-phase modulation in long-geometry optical waveguides,” Phys. Rev. A 23(3), 1266–1270 (1981). [CrossRef]
36. B. Zysset, P. Beaud, and W. Hodel, “Generation of optical solitons in the wavelength region 1.37–1.49 μm,” Appl. Phys. Lett. 50(16), 1027–1029 (1987). [CrossRef]
37. F. Xing, Z. B. Liu, Z. C. Deng, X. T. Kong, X. Q. Yan, X. D. Chen, Q. Ye, C. P. Zhang, Y. S. Chen, and J. G. Tian, “Sensitive real-time monitoring of refractive indexes using a novel graphene-based optical sensor,” Sci Rep 2, 908 (2012). [CrossRef] [PubMed]
38. X. M. Liu, D. D. Han, Z. P. Sun, C. Zeng, H. Lu, D. Mao, Y. D. Cui, and F. Q. Wang, “Versatile multi-wavelength ultrafast fiber laser mode-locked by carbon nanotubes,” Sci Rep 3, 2718 (2013). [PubMed]
