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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 14 — Jul. 15, 2013
  • pp: 16763–16776
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Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser

Yung-Hsiang Lin, Chun-Yu Yang, Jia-Hong Liou, Chin-Ping Yu, and Gong-Ru Lin  »View Author Affiliations


Optics Express, Vol. 21, Issue 14, pp. 16763-16776 (2013)
http://dx.doi.org/10.1364/OE.21.016763


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Abstract

A photonic crystal fiber (PCF) with high-quality graphene nano-particles uniformly dispersed in the hole cladding are demonstrated to passively mode-lock the erbium-doped fiber laser (EDFL) by evanescent-wave interaction. The few-layer graphene nano-particles are obtained by a stabilized electrochemical exfoliation at a threshold bias. These slowly and softly exfoliated graphene nano-particle exhibits an intense 2D band and an almost disappeared D band in the Raman scattering spectrum. The saturable phenomena of the extinction coefficient β in the cladding provides a loss modulation for the intracavity photon intensity by the evanescent-wave interaction. The evanescent-wave mode-locking scheme effectively enlarges the interaction length of saturable absorption with graphene nano-particle to provide an increasing transmittance ΔT of 5% and modulation depth of 13%. By comparing the core-wave and evanescent-wave mode-locking under the same linear transmittance, the transmittance of the graphene nano-particles on the end-face of SMF only enlarges from 0.54 to 0.578 with ΔT = 3.8% and the modulation depth of 10.8%. The evanescent wave interaction is found to be better than the traditional approach which confines the graphene nano-particles at the interface of two SMF patchcords. When enlarging the intra-cavity gain by simultaneously increasing the pumping current of 980-nm and 1480-nm pumping laser diodes (LDs) to 900 mA, the passively mode-locked EDFL shortens its pulsewidth to 650 fs and broadens its spectral linewidth to 3.92 nm. An extremely low carrier amplitude jitter (CAJ) of 1.2-1.6% is observed to confirm the stable EDFL pulse-train with the cladding graphene nano-particle based evanescent-wave mode-locking.

© 2013 Optical Society of America

1. Introduction

In addition to the graphene based saturable absorbers [1

1. Q. L. Bao, H. Zhang, 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]

9

9. G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]

] for the passively mode-locked fiber lasers, the graphene nano-particle has shown its potential to be the mode-locker for erbium-doped fiber lasers (EDFLs) [10

10. G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]. Because graphite is easily cleaved due to the weak coupling of van der Waals forces between each graphene plane, a convenient polishing method to triturate the graphene nano-particle from highly oriented pyrolytic graphite (HOPG) foil was demonstrated previously [10

10. G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

, 11

11. Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

]. The graphene nano-particle exhibits similar optical properties with graphene, such as fast carrier relaxation time, wideband absorption and superior thermal conductivity etc. In addition to reduce the size of graphene nano-particle for detuning the coverage ratio at the interacting cross-section area, the effect of layer number of the graphite or graphene on the saturable absorption was investigated to optimize the mode-locking performance. Bao et al. have studied that reducing the graphene layer number can enhance the mode-locking force of graphene saturable absorber, as attributed to the enlarged modulation depth and decreased linear absorbance. A stabilized and shortened mode-locking pulse is obtained by using an atomic-layer graphene as compared to that by a multilayer graphene [1

1. Q. L. Bao, H. Zhang, 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]

, 13

13. Q. L. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Y. Tang, and K. P. Loh, “Monolayer graphene as a saturable absorber in a mode-locked laser,” Nano Res. 4(3), 297–307 (2011). [CrossRef]

].

At current stage, the size shrinkage, layer number reduction and uniformity of graphene nano-particle can only be roughly controlled by the polishing conditions of mechanical trituration. In the case of fabricating graphene nano-particle saturable absorber, the few-layer and multilayer graphene nano-particles are co-existed after the mechanical polish process. Therefore, the layer number of graphene nano-particle must be decreased when considering it as a saturable absorber. Besides, the uniformity of graphene nano-particles is mandatory to precisely control the coverage ratio as well as linear insertion loss on the fiber end-face. These drawbacks were left the unsolved issues up to now. Recently, the electrochemical exfoliation has emerged as a simple solution-processed fabrication for obtaining few-layer graphene nano-particles from the graphite foil [14

14. C. Y. Su, A. Y. Lu, Y. Xu, F. R. Chen, A. N. Khlobystov, and L. J. Li, “High-quality thin graphene films from fast electrochemical exfoliation,” ACS Nano 5(3), 2332–2339 (2011). [CrossRef] [PubMed]

16

16. D. Wei, L. Grande, V. Chundi, R. White, C. Bower, P. Andrew, and T. Ryhänen, “Graphene from electrochemical exfoliation and its direct applications in enhanced energy storage devices,” Chem. Commun. (Camb.) 48(9), 1239–1241 (2012). [CrossRef] [PubMed]

]. Su et al. used the electrochemical exfoliation to fabricate a large-scale and few-layer graphene sheet by setting the graphite foil as an electrode under a bias voltage of + 10V [14

14. C. Y. Su, A. Y. Lu, Y. Xu, F. R. Chen, A. N. Khlobystov, and L. J. Li, “High-quality thin graphene films from fast electrochemical exfoliation,” ACS Nano 5(3), 2332–2339 (2011). [CrossRef] [PubMed]

]. However, the nonuniform graphene sheet with numerous structural defects is caused by the fast exfoliation mechanism at such high bias voltage.

On the other hand, most of the carbon based saturable absorbers [2

2. 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, 3874–3899 (2009).

8

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]

] were sandwiched between two fiber patchcords to provide the loss modulation [2

2. 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, 3874–3899 (2009).

8

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]

, 17

17. 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]

33

33. K. N. Cheng, Y. H. Lin, and G.-R. Lin, “Single- and double-walled CNT based saturable absorbers for passively mode-locking erbium-doped fiber laser,” Laser Phys. 23(4), 045105 (2013). [CrossRef]

]. Sun et al. set a graphene-polymer composite obtained by a wet-chemical method between two fiber patchcords to passively mode-lock the EDFL with a 460-fs pulsewidth [4

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]

]. Zhang et al. placed the atomic-layer graphene between two SMF patchcords to generate a mode-locked EDFL soliton with 30-nm wavelength tunability [18

18. H. Zhang, D. Y. Tang, L. M. Zhao, Q. 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]

]. Similar wavelength tunability at C- and L-bands with the insertion of a space between SMF patchcord connectors has also been reported in other kinds of fiber lasers [19

19. G.-R. Lin, J.-Y. Chang, Y.-S. Liao, and H.-H. Lu, “L-band erbium-doped fiber laser with coupling-ratio controlled wavelength tunability,” Opt. Express 14(21), 9743–9749 (2006). [CrossRef] [PubMed]

, 20

20. G.-R. Lin and I.-H. Chiu, “Femtosecond wavelength tunable semiconductor optical amplifier fiber laser mode-locked by backward dark-optical-comb injection at 10 GHz,” Opt. Express 13(22), 8772–8780 (2005). [CrossRef] [PubMed]

]. This patchcord/absorber/patchcord scheme is compact; however, the interaction is limited by the saturable absorber thickness. Increasing the thickness of saturable absorber in this scheme inevitably leads to a large insertion loss to increase the mode-locking threshold [34

34. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

], and the thermal damage of graphene materials caused by the directly propagated laser beam also set a constrain for high-power operation. More recently, the EDFL passively mode-locked by evanescent-wave saturable absorption with graphene is developed to concurrently solve the reaction length and thermal damage problems [35

35. 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]

40

40. J. Lee, J. Koo, P. Debnath, Y.-W. Song, and J. H. Lee, “A Q-switched, mode-locked fiber laser using a graphene oxide-based polarization sensitive saturable absorber,” Laser Phys. Lett. 10(3), 035103 (2013). [CrossRef]

]. Song et al. reported the 1.3-ps pulsewidth with the evanescent-field saturable absorption by graphene at cladding region, which endures an intracavity power of up to 21 dBm without damaging the graphene [35

35. 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]

]. Luo et al. demonstrated a graphene-deposited fiber taper to obtain the multi-wavelength mode-locked EDFL with 8.8-ps pulsewidth [37

37. Z. Q. Luo, J. Z. Wang, M. Zhou, H. Y. Xu, Z. P. Cai, and C. C. Ye, “Multiwavelength mode-locked erbium-doped fiber laser based on the interaction of graphene and fiber-taper evanescent field,” Laser Phys. Lett. 9(3), 229–233 (2012). [CrossRef]

]. Choi et al. utilized a graphene-injected hollow optical fiber (HOF) to achieve the evanescent-wave mode-locking of EDFL with 510-fs pulsewidth [38

38. S. Y. Choi, D. K. Cho, Y. W. Song, K. Oh, K. Kim, F. Rotermund, and D. I. Yeom, “Graphene-filled hollow optical fiber saturable absorber for efficient soliton fiber laser mode-locking,” Opt. Express 20(5), 5652–5657 (2012). [CrossRef] [PubMed]

]. Although the reacting length is lengthened, the interaction area is still limited on one-side of the taper fiber or on the single hole of HOF. This greatly lengthens the device and causes additional cavity loss.

In this work, the high-quality few-layer graphene nano-particle is obtained by using the stabilized electrochemical exfoliation at a threshold bias condition. The decreasing exfoliation rate significantly and precisely controls the layer number of the graphene nano-particles. Raman scattering spectroscopy is performed to determine the structural quality and layer number of graphene nano-particle. By syphoning the graphene nano-particles into a multi-core photonic crystal fiber (PCF), the evanescent-wave of the EDFL interacts with the graphene nano-particle uniformly in the hole cladding region. The multi-core PCF contains more graphene nano-particles in a shorter segment, which can strengthen the nonlinear saturable absorption as compared to that in a one-core HOF. By inserting the graphene nano-particle doped PCF with an interaction length of 200 μm, the evanescent-wave mode-locking of EDFL with low pumping threshold, sub-picosecond pulsewidth and ultra-low carrier amplitude jitter is demonstrated.

2. Experiment setup

Figure 1
Fig. 1 The flow chart of electrochemical exfoliation and extraction of graphene nano-particle into the PCF. The inset is the SEM image of graphene nano-particles.
illustrates the flow chart of electrochemical exfoliation, centrifugation, and syphoning of the graphene nano-particles into the PCF. In the electrochemical exfoliation, the HOPG foil and a Pt wire are respectively served as the anode and the cathode in the electrolyte of sulfuric acid aqueous solution, which is prepared by diluting 96% sulfuric acid and 100 mL of deionized water. The exfoliation process is operated by applying different DC bias voltages of + 3 and + 6V. When operating at a bias of + 6V, the HOPG foil is rapidly split and dissociated into small particles. The roughly exfoliated graphite stacks are observed right after turn-on the DC power supply. In contrast, the operation under a threshold bias of + 3V spends more than 20 sec to start the exfoliation after turn-on the DC power supply, and the exfoliated graphene nano-particles are more delicate. Subsequently, the graphene nano-particle aqueous solution is filtered to remove the large graphene sheets by a porous filter, and the percolated graphene nano-particles are preserved in acetone solution. After the ultrasonic agitation of graphene nano-particle contained in acetone solution for 10 min, the centrifugation of graphene nano-particle solution can separate small particles as the supernatants but deposit the large particles in the bottom.

The supernatants are syphoned into the PCF (NKT, LMA-10) to make the graphene nano-particles dispersed in the PCF. To avoid the modification of guiding mode in the PCF filled with the acetone solution in the hole cladding region, the passive mode-locking of EDFL is performed after evaporating the acetone solution in an oven at 70° for 1 hour. The SEM image confirms the sizes of graphene nano-particles are around 400 nm. The slowly exfoliated graphene nano-particle obtained with low bias voltage is the preferred candidate to be used in the EDFL system. In addition, the centrifugation is a necessary process to collect the uniform and delicate graphene nano-particles after the electrochemical exfoliation, because the few-layer graphene nano-particle with small size induces low insertion loss but high modulation depth. Afterwards, the PCF with dispersed graphene nano-particles is spliced with another PCF. The evanescent-wave interaction improves the nonlinear interaction length and increases the tolerance of high intra-cavity power. The advantages of the fabrication process are simple, convenient and high reproducibility.

Figure 2
Fig. 2 The experimental setup of the passively mode-locked EDFL. Inset: the photographs of the PCF (left: top-view, right: side-view).
demonstrates the experimental setup of the passively mode-locked EDFL. This system utilizes an erbium-doped fiber (EDF, nLIGHT Liekki Er80-8/125) as the gain medium, which is bi-directionally pumped by a 980-nm laser diode (LD, forward) and a 1480-nm LD (backward) through two wavelength-division multiplexers (WDMs). The circulated direction is determined by an isolator and the intra-cavity polarization is controlled by a polarization controller (PC). A 95/5 coupler is inserted to provide 5% output and feedback 95% of intra-cavity power. The EDFL consists of a 2-m long EDF with dispersion coefficient β2,EDF of −20 ps2/km, and a 6.2-m long SMF with β2,SMF of −20 ps2/km [12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

, 41

41. G.-R. Lin, C. L. Pan, and Y. T. Lin, “Self-steepening of prechirped amplified and compressed 29-fs fiber laser pulse in large-mode-area erbium-doped fiber amplifier,” J. Lightwave Technol. 25(11), 3597–3601 (2007). [CrossRef]

]. In the inset of Fig. 2, the photographs show the optical microscopy images of PCF. The top-view of PCF indicates that the hole diameter is 3.1 μm with the spacing between each hold of 7.1 μm. The length of PCF is 200 μm as shown in the side-view image. The dispersion coefficient β2,PCF of the PCF is about −40 ps2/km.

3. Results and discussions

Figure 3(a)
Fig. 3 Raman spectra of the HOPG foil and the electrochemically exfoliated graphene nano-particles operated under bias voltages of + 6 and + 3V.
compares the Raman scattering spectra of the original HOPG foil and the electrochemically exfoliated graphene nano-particles obtained at bias voltages of + 3 and + 6V. The HOPG foil exhibits two prominent Raman signals at 1580 and 2730 cm−1, which represent G band and 2D band originated from the sp2 carbon network and the second-order double resonance [11

11. Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

,12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

,42

42. 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]

45

45. O. Frank, M. Mohr, J. Maultzsch, C. Thomsen, I. Riaz, R. Jalil, K. S. Novoselov, G. Tsoukleri, J. Parthenios, K. Papagelis, L. Kavan, and C. Galiotis, “Raman 2D-band splitting in graphene: theory and experiment,” ACS Nano 5(3), 2231–2239 (2011). [CrossRef] [PubMed]

]. As the graphene layer number increases to form graphite, the 2D band intensity significantly reduces with a broadening linewidth and an asymmetric spectral shape. The divided phonon branches contribute different phonon frequencies that could lead to the splitting of 2D band [42

42. 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]

]. After the electrochemical exfoliation at a bias of + 6V, the exfoliated graphene nano-particle shows a relatively broadened G band and an attenuated 2D band. To quantify, the intensity ratio of 2D band over G band (I2D/IG) is about 0.35. In addition, a distinct D band with an intensity ratio of D band over G band (ID/IG) of 1.57 is observed, which is mainly caused by the structural defects occurred outside the graphene nano-particle [11

11. Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

, 12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]. The defects are inevitably generated outside the graphene nano-particle when the graphene nano-particles are rapidly exfoliated from the HOPG foil, such as cracks, vacancies, stretches and bending, as shown in Fig. 4
Fig. 4 The schematic diagrams of the electrochemically exfoliated graphene nano-particles with bias voltages of + 6 and + 3V.
.

Most of the defects are tensile strains after the electrochemical exfoliation, which lead to the red-shift of the Raman scattering peak wavenumber at 2D band [43

43. Z. H. Ni, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng, and Z. X. Shen, “Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening,” ACS Nano 2(11), 2301–2305 (2008). [CrossRef] [PubMed]

, 45

45. O. Frank, M. Mohr, J. Maultzsch, C. Thomsen, I. Riaz, R. Jalil, K. S. Novoselov, G. Tsoukleri, J. Parthenios, K. Papagelis, L. Kavan, and C. Galiotis, “Raman 2D-band splitting in graphene: theory and experiment,” ACS Nano 5(3), 2231–2239 (2011). [CrossRef] [PubMed]

]. In contrast, the G band of the graphene nano-particle obtained under the exfoliated operation with bias voltage of + 3V becomes intense and sharp, and the 2D band increases it intensity with an enlarging I2D/IG ratio of 0.54, whereas the D band is approximately disappeared to show a dramatically decreased ID/IG ratio of 0.12. This observation indicates that the crystalline quality of graphene nano-particle can be improved and the size (or layer number) of graphene nano-particle is also reduced by decreasing the bias voltage, as shown in Fig. 4. Because the graphene nano-particles are slowly and softly exfoliated from the graphite foil under the stable exfoliation bias of + 3V, as compared to the operation of + 6V bias voltage. To be the saturable absorber, the large graphene nano-particle with plenty of defects obtained by the operation of + 6V bias voltage is not a good candidate due to the insufficient modulation depth and the enlarged absorption loss [12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]. Therefore, the electrochemical exfoliation operated under + 3V bias voltage is suitable to fabricate few-layer graphene nano-particle with better crystalline quality.

The schematic diagram of the evanescent-wave mode-locking for inducing the pulse formation with graphene nano-particles dispersed in hole-cladding region of the PCF is shown in Fig. 7
Fig. 7 The schematic diagram of evanescent-wave mode-locked pulse propagation through the PCF doped with graphene nano-particles in hole-cladding region.
. Assuming that the graphene nano-particle dispersed in the PCF has a nonlinear absorption coefficient given by
αnon,G=α0,G1+Ie,t/Ie,satIe,t=Ie,sat(α0,Gαnon,G1)
(2)
where αo,G is the nonlinear absorption component, Ie,t represents the evanescent-wave intensity and Ie,sat denotes the saturable intensity of graphene nano-particles.

Within the hole-cladding region of the PCF, the evanescent-wave exponentially decays with the radial distance (x) away from the core/cladding interface, and the evanescent-wave intensity can be described as [46

46. E. Hecht, Optics (Addison Wesley, 4th Edition).

]
Ie,t=I0e2βx,
(3)
where I0 and β denote the intracavity pulse intensity and the extinction coefficient factor. With the aid of graphene nano-particles, the extinction coefficient factor is re-written as:
β=2πncλ(sin2θi(nc/ni)21)1/2=2πλ{ni2[nc0+ncIe,sat(α0,Gαnon,G(It)1)]2}1/2,
(4)
where ni and nc are the core refractive index and the effective cladding refractive index. nc is determined by the dispersed graphene nano-particles which can be expressed as nc0 + nc.It with nc representing the nonlinear refractive index. According to the Z-scan measurement of the graphene materials given by Zhang et al., the nonlinear refractive index nc also exhibits an intensity dependent behavior, that is, the nc decreases and eventually saturates to a value of 6x10−6 (W/cm2)−1 [47

47. H. Zhang, S. Virally, Q. L. Bao, L. K. Ping, S. Massar, N. Godbout, and P. Kockaert, “Z-scan measurement of the nonlinear refractive index of graphene,” Opt. Lett. 37(11), 1856–1858 (2012). [CrossRef] [PubMed]

]. This consequently results in the saturable phenomena for both the extinction coefficient β and the field confinement factor Γ given by [48

48. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics. (Wiley, New York, 1991).

]:

Γ=0d/2Ecore2(x)dx0d/2Ecore2(x)dx+d/2Ecladding2(x)dx.
(5)

The saturable phenomena of the extinction coefficient β determines the decay length (1/2β) of the evanescent wave, which results in a Kerr-lens like refractive index change along the transverse direction of the PCF fiber, thus providing a loss modulation for the intracavity photon intensity. According the schematic diagram shown in Fig. 8
Fig. 8 Schematic diagram of evanescent field modulation with varied extinction coefficient.
, the decay length decreases to perform the low evanescent-wave intensity when β increases, whereas the decay length increases with reducing β value to broaden the evanescent-wave field. Such a phenomenon occurs back and forth by the saturable-absorption of graphene nano-particles distributed in the hole-cladding region of the PCF.

To perform the evanescent-wave interaction for starting the passively mode-locked EDFL, the graphene nano-particle injected hollow optical fiber (HOF) [38

38. S. Y. Choi, D. K. Cho, Y. W. Song, K. Oh, K. Kim, F. Rotermund, and D. I. Yeom, “Graphene-filled hollow optical fiber saturable absorber for efficient soliton fiber laser mode-locking,” Opt. Express 20(5), 5652–5657 (2012). [CrossRef] [PubMed]

] with a length of 59 mm has been inserted into the EDFL ring cavity. In our case, because more graphene nano-particles can be siphoned into the multi-core PCF to enhance the nonlinear interaction, the PCF with a length of only 200-μm is required to produce the evanescent-wave interaction with ΔT of 5%. The attenuation loss caused by the PCF can certainly be decreased and a large GDD caused by the PCF can also be avoided by shortening the PCF, simultaneously. However, there is a trade-off between the degradation of saturable absorption and aforementioned effects. Figure 9(a)
Fig. 9 (a) Pout-Pin curves and (b) the Gain curves of the EDFA under different pumping currents.
and 9(b) show the Pout vs. Pin transfer response and the power gain curves of the EDFA with increasing both the pumping currents of 980-nm LD and 1480 nm-LD from 700 to 900 mA. A continuous-wave laser with a wavelength of 1570 nm is passing through the EDFA to measure the power gain at different pumping currents. The pumping power of 980-nm LD enlarges from 235 to 290 mW, and the pumping power of 1480-nm LD enlarges from 153 to 200 mW by increasing the pumping current from 700 to 900 mA. The Gain curves of the EDFA are simulated by G = G0/(1 + Pin/Psat), where G0 is the small signal gain, Pin and Psat are the input power and the saturated power of EDFA, respectively. G0 rises from 8.42 to 8.62, and Psat enlarges from 0.55 to 0.65 with increasing the pumping currents of 980-nm LD and 1480-nm LD simultaneously.

Figure 10(a)
Fig. 10 (a) The autocorrelation traces and (b) optical spectra of the passively mode-locked EDFLs under different pumping current.
and 10(b) depict the autocorrelation traces and the optical spectra of the passively mode-locked EDFLs under different pumping power. The autocorrelation traces and the optical spectra are measured by an autocorrelator (Femtochrome, FR-103XL) and an optical spectrum analyzer (Ando, AQ6317B), respectively. By simultaneously increasing the pumping currents of 980-nm and 1480-nm LDs from 700 to 900 mA, the enhanced net optical gain increases the synchronous oscillating modes [34

34. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

]. As a result, the passively mode-locked EDFL pulsewidth slightly shrinks from 668 to 650 fs with the corresponding spectral full-width at half maximum (FWHM) broadened from 3.75 to 3.92 nm. The wavelength of the EDFL in all cases remains unchanged at 1567.6 nm. The time-bandwidth products (TBPs) of the EDFLs are around 0.315, and the group delay dispersion (GDD) of the laser cavity is approximately −0.164 ps2/km. In the anomalous dispersion condition, the soliton pulse formation is periodically perturbed by the linear effect of GDD and the nonlinear effect of self-phase modulation (SPM) [12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

, 49

49. G.-R. Lin, I.-H. Chiu, and M. C. Wu, “1.2-ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60-ps backward dark-optical comb injection and soliton compression,” Opt. Express 13(3), 1008–1014 (2005). [CrossRef] [PubMed]

].

The soliton mode-locking operation and the occurrence of Kelly sidebands on the shoulder of optical spectra are observed in the EDFL system. To confirm the optimized performances of passively mode-locked EDFL started by the graphene nano-particles doped in PCF, both the pulse shape and the optical spectrum are simulated by using Haus master equation with the experimentally obtained parameters. The master equation is given by [12

12. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]:
TRTA(T,t)=[G01+|A|2/PG,satl+Dg2t2qnon(T,t)1+|A|2/PsatiD2t2+iδ|A|2]A(T,t),
(6)
where TR and A(T,t) represent the round-trip time and the pulse amplitude, G0 and l denote the cavity gain and the cavity loss, Dg is the gain dispersion, D, qnon, and δ denote the cavity dispersion, the saturable loss and the self-phase modulation (SPM) coefficient, respectively. The round-trip circulation number of the equation is set up to 10000 for stabilizing the pulse formation. Figure 11(a)
Fig. 11 (a) The simulated autocorrelation traces and (b) optical spectra of the passively mode-locked EDFLs under different pumping current.
and 11(b) demonstrate the simulated autocorrelation traces and the optical spectra under different pumping currents. By simultaneously increasing the pumping current of 980-nm and 1480-nm LDs from 700 to 900 mA, the simulated pulse shapes show the pulsewidth shortening from 683 to 655 fs, with the spectral FWHM broadening from 3.71 to 3.86 nm, which are well correlated with the experimental results.

At last, the stabilization of the evanescent-wave mode-locking performance is monitored by characterizing the peak power fluctuation among the adjacent mode-locked pulses. The inset of Fig. 12
Fig. 12 The CAJ values and the oscilloscope traces of the passively mode-locked EDFLs under different pumping currents.
exhibits the oscilloscope traces of the passively mode-locked EDFLs under different pumping currents. The repetition time of the EDFL is 40 ns with the corresponding repetition rate of 25 MHz. By using the evanescent-wave mode-locking, the location of graphene nano-particles in the hole-cladding region of the PCF can ensure the high intracavity power in the core region without suffering from any thermal dissipation, which generates a highly stabilized mode-locking of EDFL at a larger output power. The quality of pulse-amplitude equalization can be determined by calculating the carrier amplitude jitter (CAJ) of the measured mode-locked pulse-train in time domain, which is defined as a ratio of the standard deviation (σ) on peak pulse intensity to the average pulse intensity (Iave), CAJ = (σ/Iave)x100% [50

50. G.-R. Lin, J. J. Kang, and C. K. Lee, “High-order rational harmonic mode-locking and pulse-amplitude equalization of SOAFL via reshaped gain-switching FPLD pulse injection,” Opt. Express 18(9), 9570–9579 (2010). [CrossRef] [PubMed]

]. The extremely low CAJ values of around 1.23%~1.66% are obtained for different pumping cases and shown in Fig. 12, indicating a very small peak power fluctuation for the evanescent-wave mode-locked EDFL started with the graphene nano-particles doped in the hole-cladding region of the PCF. The operating lifetime of such an evanescent-wave mode-locked EDFL can be stably operated up to 12 hours, which is at least 1.5 times longer than the same system using the core-region mode-locking with same graphene nano-particle based saturable absorber. This observation confirms the stabilization of the evanescent-wave mode-locked EDFL.

4. Conclusion

Acknowledgment

This work was supported by National Science Council and National Taiwan University under grants NSC101-2622-E-002-009-CC2, NSC101-2221-E-002-071-MY3 and NTU102R89083.

References and links

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2.

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, 3874–3899 (2009).

3.

H. Zhang, Q. L. Bao, D. Y. Tang, L. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fber 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.

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]

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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]

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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]

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G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]

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G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

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Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

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Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

13.

Q. L. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Y. Tang, and K. P. Loh, “Monolayer graphene as a saturable absorber in a mode-locked laser,” Nano Res. 4(3), 297–307 (2011). [CrossRef]

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C. Y. Su, A. Y. Lu, Y. Xu, F. R. Chen, A. N. Khlobystov, and L. J. Li, “High-quality thin graphene films from fast electrochemical exfoliation,” ACS Nano 5(3), 2332–2339 (2011). [CrossRef] [PubMed]

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J. Lu, J.-X. Yang, J. Wang, A. Lim, S. Wang, and K. P. Loh, “One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids,” ACS Nano 3(8), 2367–2375 (2009). [CrossRef] [PubMed]

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D. Wei, L. Grande, V. Chundi, R. White, C. Bower, P. Andrew, and T. Ryhänen, “Graphene from electrochemical exfoliation and its direct applications in enhanced energy storage devices,” Chem. Commun. (Camb.) 48(9), 1239–1241 (2012). [CrossRef] [PubMed]

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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]

18.

H. Zhang, D. Y. Tang, L. M. Zhao, Q. 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]

19.

G.-R. Lin, J.-Y. Chang, Y.-S. Liao, and H.-H. Lu, “L-band erbium-doped fiber laser with coupling-ratio controlled wavelength tunability,” Opt. Express 14(21), 9743–9749 (2006). [CrossRef] [PubMed]

20.

G.-R. Lin and I.-H. Chiu, “Femtosecond wavelength tunable semiconductor optical amplifier fiber laser mode-locked by backward dark-optical-comb injection at 10 GHz,” Opt. Express 13(22), 8772–8780 (2005). [CrossRef] [PubMed]

21.

Q. L. Bao, H. Zhang, J. X. 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(5), 782–791 (2010). [CrossRef]

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H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. L. Bao, and K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fber laser,” Appl. Phys. Lett. 96(11), 111112 (2010). [CrossRef]

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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). [CrossRef] [PubMed]

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H. Kim, J. H. Cho, S. Y. Jang, and Y. W. Song, “Deformation immunized optical deposition of graphene for ultrafast pulsed lasers,” Appl. Phys. Lett. 98(2), 021104 (2011). [CrossRef]

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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]

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J.-C. Chiu, C.-M. Chang, B.-Z. Hsieh, S.-C. Lin, C.-Y. Yeh, G.-R. Lin, C.-K. Lee, J.-J. Lin, and W.-H. Cheng, “Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber,” Opt. Express 19(5), 4036–4041 (2011). [CrossRef] [PubMed]

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J. Sotor, G. Sobon, and K. M. Abramski, “Scalar soliton generation in all-polarization-maintaining, graphene mode-locked fiber laser,” Opt. Lett. 37(11), 2166–2168 (2012). [CrossRef] [PubMed]

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K. N. Cheng, Y. H. Lin, S. Yamashita, and G.-R. Lin, “Harmonic order dependent pulsewidth shortening of a passively mode-locked fiber laser with carbon nanotube saturable absorber,” IEEE Photon. J 4(5), 1542–1552 (2012). [CrossRef]

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G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]

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G. Sobon, J. Sotor, and K. M. Abramski, “Passive harmonic mode-locking in Er-doped fiber laser based on graphene saturable absorber with repetition rates scalable to 2.22 GHz,” Appl. Phys. Lett. 100(16), 161109 (2012). [CrossRef]

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J. Du, S. M. Zhang, H. F. Li, Y. C. Meng, X. L. Li, and Y. P. Hao, “L-Band passively harmonic mode-locked fiber laser based on a graphene saturable absorber,” Laser Phys. Lett. 9, 896–900 (2012).

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Y. C. Meng, S. Zhang, X. Li, H. Li, J. Du, and Y. P. Hao, “Multiple-soliton dynamic patterns in a graphene mode-locked fiber laser,” Opt. Express 20(6), 6685–6692 (2012). [CrossRef] [PubMed]

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Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

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G.-R. Lin, I.-H. Chiu, and M. C. Wu, “1.2-ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60-ps backward dark-optical comb injection and soliton compression,” Opt. Express 13(3), 1008–1014 (2005). [CrossRef] [PubMed]

50.

G.-R. Lin, J. J. Kang, and C. K. Lee, “High-order rational harmonic mode-locking and pulse-amplitude equalization of SOAFL via reshaped gain-switching FPLD pulse injection,” Opt. Express 18(9), 9570–9579 (2010). [CrossRef] [PubMed]

OCIS Codes
(140.3510) Lasers and laser optics : Lasers, fiber
(140.4050) Lasers and laser optics : Mode-locked lasers
(140.7090) Lasers and laser optics : Ultrafast lasers
(160.4236) Materials : Nanomaterials
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: April 15, 2013
Revised Manuscript: May 30, 2013
Manuscript Accepted: May 30, 2013
Published: July 5, 2013

Citation
Yung-Hsiang Lin, Chun-Yu Yang, Jia-Hong Liou, Chin-Ping Yu, and Gong-Ru Lin, "Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser," Opt. Express 21, 16763-16776 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-14-16763


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References

  1. Q. L. Bao, H. Zhang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
  2. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, A. C. Ferrari, “Nanotube-polymer composites for ultrafast photonics,” Adv. Mater. 21, 3874–3899 (2009).
  3. H. Zhang, Q. L. Bao, D. Y. Tang, L. Zhao, K. P. Loh, “Large energy soliton erbium-doped fber 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, 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, A. C. Ferrari, “Sub 200 fs pulse generation from a graphene mode-locked fiber laser,” Appl. Phys. Lett. 97(20), 203106 (2010). [CrossRef]
  6. Y. M. Chang, H. Kim, J. H. Lee, 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]
  7. A. Martinez, K. Fuse, 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, 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. G. Sobon, J. Sotor, K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]
  10. G.-R. Lin, Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]
  11. Y. H. Lin, G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]
  12. Y. H. Lin, G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]
  13. Q. L. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Y. Tang, K. P. Loh, “Monolayer graphene as a saturable absorber in a mode-locked laser,” Nano Res. 4(3), 297–307 (2011). [CrossRef]
  14. C. Y. Su, A. Y. Lu, Y. Xu, F. R. Chen, A. N. Khlobystov, L. J. Li, “High-quality thin graphene films from fast electrochemical exfoliation,” ACS Nano 5(3), 2332–2339 (2011). [CrossRef] [PubMed]
  15. J. Lu, J.-X. Yang, J. Wang, A. Lim, S. Wang, K. P. Loh, “One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids,” ACS Nano 3(8), 2367–2375 (2009). [CrossRef] [PubMed]
  16. D. Wei, L. Grande, V. Chundi, R. White, C. Bower, P. Andrew, T. Ryhänen, “Graphene from electrochemical exfoliation and its direct applications in enhanced energy storage devices,” Chem. Commun. (Camb.) 48(9), 1239–1241 (2012). [CrossRef] [PubMed]
  17. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, 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]
  18. H. Zhang, D. Y. Tang, L. M. Zhao, Q. Bao, K. P. Loh, B. Lin, 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]
  19. G.-R. Lin, J.-Y. Chang, Y.-S. Liao, H.-H. Lu, “L-band erbium-doped fiber laser with coupling-ratio controlled wavelength tunability,” Opt. Express 14(21), 9743–9749 (2006). [CrossRef] [PubMed]
  20. G.-R. Lin, I.-H. Chiu, “Femtosecond wavelength tunable semiconductor optical amplifier fiber laser mode-locked by backward dark-optical-comb injection at 10 GHz,” Opt. Express 13(22), 8772–8780 (2005). [CrossRef] [PubMed]
  21. Q. L. Bao, H. Zhang, J. X. Yang, S. Wang, D. Y. Tang, R. Jose, S. Ramakrishna, C. T. Lim, K. P. Loh, “Graphene-polymer nanofiber membrane for ultrafast photonics,” Adv. Funct. Mater. 20(5), 782–791 (2010). [CrossRef]
  22. H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. L. Bao, K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fber laser,” Appl. Phys. Lett. 96(11), 111112 (2010). [CrossRef]
  23. A. Martinez, K. Fuse, B. Xu, 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). [CrossRef] [PubMed]
  24. H. Kim, J. H. Cho, S. Y. Jang, Y. W. Song, “Deformation immunized optical deposition of graphene for ultrafast pulsed lasers,” Appl. Phys. Lett. 98(2), 021104 (2011). [CrossRef]
  25. A. Martinez, K. Fuse, S. Yamashita, “Mechanical exfoliation of graphene for the passive mode-locking of fiber lasers,” Appl. Phys. Lett. 99(12), 121107 (2011). [CrossRef]
  26. J.-C. Chiu, C.-M. Chang, B.-Z. Hsieh, S.-C. Lin, C.-Y. Yeh, G.-R. Lin, C.-K. Lee, J.-J. Lin, W.-H. Cheng, “Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber,” Opt. Express 19(5), 4036–4041 (2011). [CrossRef] [PubMed]
  27. J. Sotor, G. Sobon, K. M. Abramski, “Scalar soliton generation in all-polarization-maintaining, graphene mode-locked fiber laser,” Opt. Lett. 37(11), 2166–2168 (2012). [CrossRef] [PubMed]
  28. K. N. Cheng, Y. H. Lin, S. Yamashita, G.-R. Lin, “Harmonic order dependent pulsewidth shortening of a passively mode-locked fiber laser with carbon nanotube saturable absorber,” IEEE Photon. J 4(5), 1542–1552 (2012). [CrossRef]
  29. G. Sobon, J. Sotor, K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]
  30. G. Sobon, J. Sotor, K. M. Abramski, “Passive harmonic mode-locking in Er-doped fiber laser based on graphene saturable absorber with repetition rates scalable to 2.22 GHz,” Appl. Phys. Lett. 100(16), 161109 (2012). [CrossRef]
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